First off, this is a really good idea, and I like what you have so far. I don't know how much (if any) of this you want to use, but I'm sort of writing independently of what you've already written.
Throughout I'm going to assume open or semi-open set-ups, because I've never played or tried to design (non-bastard) closed set-ups, and I'm not really sure what makes closed set-ups fun for people (I don't mean that as in "you're crazy if you like closed set-ups", just literally I'm not sure what makes them fun for people), so other people would have much better advice on that than me.
I think there are n different kinds of set-ups you can design, where n is a number I'll know when I'm done writing this post.
Unique Mechanic/Gameplay
The first kind of set-up you can have is one which revolves around some unique, original mechanic (usually). Examples of this would be Yoshi's Island (RMM21), ADK's Insomnia (M78), silverspawn's Duel Mafia (M83), or Paris Mafia (M66). In each of these cases, there's some unique mechanic that the game revolves around. These set-ups usually tend to be open, because players need access to information about the mechanic and also because the mechanic is usually something that makes fake claims possible or irrelevant for scum, meaning it's not necessary to make the set-up semi-open; but it's not unreasonable to have a semi-open set-up in this category.
I said usually you have a unique mechanic, but really the defining characteristic of this type is that the set-up has a unique effect on the gameplay in some way. So for example, a set-up like 8 cops vs. 5 goons would go in this category, because even though it consists only of traditional roles, the way that they're combined makes the game play in a very unusual way (scum can freely claim cop, which gives them lots of chances to throw people off; there's so many scum that they can drive wagons easily). (By the way, I've thought about this set-up a lot and I think it's heavily scum-favored, but I'm not confident about that at all).
This is generally my favorite kind of set-up I think, because it makes players really think about all of their decisions regarding their roles and the set-up. (In other words, it reduces the importance of the social deduction aspect of the game, which I personally am not really interested in.) Unfortunately, it's hard to give advice about this category, because coming up with a new original mechanic isn't really something you can do intentionally. Something might give you an idea, but there's no way (that I'm aware of) to force yourself to come up with a good idea.
However, once you've got an idea, here are some tips on how to implement it well:
1. Make sure that the unique mechanic has a both noticeable and positive impact on the gameplay. There's nothing worse than coming up with a brilliant new idea, that ends up being irrelevant to the gameplay. Actually, there is something worse, which is that your brilliant new idea actually makes the game less enjoyable. So make sure you include roles that interact well with your new mechanic, and that the decisions your new mechanic creates will be interesting and make the game more fun for everyone. And be willing to accept that maybe your idea just won't work out. Look at it objectively and don't rationalize to yourself "here's why this is actually fun", if your gut says it isn't.
2. Include only the roles you need to to make your game interesting. If your favorite role is Lightningrod, don't include a Lightningrod in your set-up just for the heck of it. If it interacts with the other roles and mechanics in an interesting way, then great, go for it. But if it's not adding any interesting decisions, simplicity is always better. I dropped the ball on this one in Yoshi's Island when I included a town Roleblocker, which I knew wasn't doing much of anything, but I had playtested the set-up so many times with it that I didn't feel comfortable cutting it. But overall the set-up would have been cleaner without it (or with something else instead of it).
3. Playtest your set-ups. Well, mafia can't really be "playtested", without actually running the set-up, but run simulations to see roughly how well each side does. The idea is that you play all the roles yourself, and whenever people would be making a "human" decision (like lynching), you make that decision randomly instead. This won't give you accurate results, because town's ability to read scum will always have an effect on the game, so ideally your results should favor scum (usually, though there could potentially be things that would change this). I think conventional wisdom is to aim for about a 70% scum win rate with random lynching, and then with human lynching it will balance out nicely. Personally I think this is too high, I usually aim for 60%, but it also can depend on the mechanics of the set-up.
(Intermission)
Before I move on to the next type of set-up, let's ask ourselves, what's the point of a semi-open set-up? Why include randomization in the set-up process? Open set-ups offer the advantage that players have all the information they need to make well-informed decisions with their roles, and closed set-ups offer the advantage that the set-up is unsolvable, giving scum the freedom to fake claim whatever they need to. Semi-open set-ups combine both of these benefits, via randomization. The mod still describes how the randomization happens, so it's still "fair" in the sense that everyone knows what they're signing up for, but the randomization gives scum chances to claim things without town necessarily being able to confirm them as lies.
Let's think about this example set-up: The mod flips a coin. The result determines which set-up is played:
Heads: 6 VT's, 1 Two-Shot Cop vs. 2 Mafia Goons
Tails: 5 VT's, 2 Two-Shot Cops vs. 2 Mafia Roleblockers
If we tried to just play the heads set-up, without the possibility of the tails set-up, we would have the problem that the cop would claim once his shots ran out, and become an IC since there's no other cops possible. However, the heads set-up avoids this problem when town doesn't know whether it's the heads or tails set-up, because now it's possible for scum to claim cop without town counterclaiming.
So by adding uncertainty to the set-up, scum has more options on how they can claim.
Random Standard Roles
This is probably the most popular type of (non-closed) set-up on f.ds, at least in terms of how many games of that type get run. This includes anything where you have standard roles that are put together according to some random process. Examples include asher9++, gkrieg's game which is currently open for sign-ups (M88), or Arch's WIFOM set-up and variants (used in M87 Portal, M48 Legend of Zelda, and M38 Super Mario).
The main ideas with these types of set-ups are that you usually like to include a variety of roles, and you usually want to leave the possibilities of various combinations as open as possible (however, that's not necessary; you can also have more limited possibilities so that you can control exactly what kinds of interactions are possible).
Coming up with your own randomization mechanics can be fun, or maybe you already know what roles you want to include but you're not sure how to put them together. In that case, here's a few randomization mechanics you can use:
1. Letter rolling: I'm going to assume you know how this works if you're reading this. If you don't, see asher9++ for an example. This mechanic gives you a very open space of possibilities for combinations of roles that can be included in the set-up. For that reason, I think asher9++ (and maybe a few of the other similar standard set-ups using this mechanic) already have this mechanic pretty well covered, since they already cover the most standard roles, and there's not much more you could hope to do with it. One advantage to this mechanic is that you can easily scale the power of scums' roles with towns' roles, based on how many PR letters town rolled. Another advantage is that the variable numbers and types of town PR's gives scum lots of room for fake claims, making it nearly impossible to "solve" the set-up mid-game. One drawback is that letters are usually all worth equal amounts (they don't have to be, but then it starts getting really complicated), so you might get a set-up that suggests docs and cops are equally valuable.
2. Point values for roles: This is a really broad mechanical idea that can be implemented in wildly different ways. The idea is usually that you assign point values to different town roles, and then use the total point values of the randomly assigned town roles as a baseline for ensuring balance. For example, in gkrieg's M88, random roles (with various point values, based on their strength) are chosen until the total points add up to 7, the idea being that 7 points-worth of town roles would generally be balanced. In M60 Kirby Mafia, I rolled numbers from 1 to 7 to determine five roles that town got, and then scum got assigned roles based on the total of the rolls (higher numbers rolled resulted in stronger roles for both town and scum). So there are lots of ways you can use this idea, but the core mechanic is to assign strength to town roles so that you can make sure they match the strength of scum's roles.
3. Randomizing from a discrete list of set-ups: This seems like a pretty basic idea but I don't think I've ever actually seen it used. The idea is that you have some list of set-ups, and you choose one at random from that list. The coin-flip example I gave earlier (where you play 6 VTs, 1 2-shot Cop vs. 2 Goons on heads and 5 VT's, 2 2-shots Cops vs. 2 Roleblockers on tails) is an example of this. You could have more than two set-ups to choose from though, but since you have to list off every possible set-up, having more than 2 possibilities can get very complex very quickly. This mechanic has both the advantage and disadvantage that it gives you an extremely narrow space of possible set-ups; basically you're using as little randomization as possible while still not being open. I think you could use this mechanic to try to fix some simple set-ups that don't quite work as open set-ups. This mechanic is useful if you want a lot of control over the set-up possibilities.
4. Randomizing town and scum roles together: Arch's WIFOM set-up has a mechanic that I really like, where you flip a coin for each of several different pairs of roles. For each coin, on heads, you include a particular town role and a particular scum role, and on tails, you don't include either of them. This means that scum will always get a chance to fake claim any town roles that aren't present in the game. It also makes balancing fairly easy, because you can pair similar-strength roles with each other so that whenever one is present, so is the other. This mechanic generally gives you a much narrower possibility space than #1 and #2, but still broader than #3. (For example, Arch's WIFOM set-up gives you 32 possible set-ups, contrasted with the usually millions or more from #1 and #2, or the 2-4 you'd probably have for #3.) So you can use this mechanic as sort of a happy medium when you want a fair amount of control over the possible set-ups, but still a good amount of variety in how many set-ups are possible.
Where there is randomness, there is probability to be done, especially on f.ds. For better or for worse, semi-open set-ups can and will be analyzed with probability, and using more complicated randomization mechanics will exaggerate both the good and the bad aspects of that. Lots of players like doing probability, so they might see it as a plus for your set-up to give them a challenging probability problem. As a designer, you might be frustrated that they're taking the fun out of your game or they're abusing math to try to break your set-up. The less "solvable" your randomization mechanics are, the more work players will have to put into "breaking" your set-up, which both means that fewer players will attempt to do so, but also that those who are successful are more heavily rewarded for using math, so they're more incentivized to do so. This is probably something you want to consider when deciding on what kinds of mechanics to use.
Generally, you want to balance for each individual possibility, not just for the overall sum of all the possibilities. For example, you could have a set-up like: Flip a coin. On heads, all town players are 2-shot Bulletproof; on tails, all scum players are 2-shot Lynchproof. In some sense, this set-up is balanced, because town and scum each have roughly a 50% chance of winning, but it also makes random chance the deciding factor, which is usually undesirable. Ideally, you would like for every single possible set-up that you could have to be balanced. So I suggest that if you're using a mechanic that gives you lots of possible set-ups, try rolling a bunch of set-ups. See if they each look reasonably well-balanced individually. I recommend rolling at least 10 times, so that you can see a variety of different kinds of set-ups you can have and make sure none of them will feel unfair for either side.
Look for good or bad combinations of roles. Doctor is a decent role, and IC is a decent role, but Doctor and IC together are a lot stronger than the sum of Doctor and IC individually. It's generally good to keep an eye out for role interactions like this. That particular example is something that should catch your attention, but probably not a big enough deal to lose sleep over. But because a lot of these kinds of set-ups allow for lots of different combinations of roles, it's a good idea to look for combinations that might make the game really lopsided or unfun.
Make sure scum can actually fake claim. Since the main benefit of semi-open set-ups is that they give scum chances to fake claim, it would defeat the point if your set-up had randomness but still forced scum to claim VT. Make sure that in most (or ideally, all) of the possible set-ups, scum will have opportunities to safely fake claim. You should also think about how much information scum needs in order to fake claim. You generally want the mod to tell them as little information about town's PR's as possible, while still ensuring that they have freedom in how the claim safely.
Simple/Newbie-Friendly
These can be either open or semi-open, but open is usually preferable. If they are semi-open, you should avoid complicated randomization mechanics that newbies would find confusing. The idea behind these set-ups is to keep the game as simple and vanilla as possible, while still making sure that a few players have night actions. The goal is to introduce new people to mafia, so if the game is totally vanilla, you don't introduce them to the idea of power roles; but with a few simple PR's, they can see how night actions work and how they affect the game, even if they themselves don't have a PR.
These sorts of set-ups can be tough to design, because you need to watch out for role interactions (like follow the cop) and make sure scum has a chance to fake claim if necessary, all while keeping everything as simple as possible and using only the bare minimum number of roles.
Some simple roles that are generally very easy for newbies to understand are: Cop, Doctor, Innocent Child; some others that are generally simple enough to include in newbie set-ups are: 1-shot Bulletproof, Roleblocker, Tracker, Watcher, Vigilante (though this is really high-pressure so I would be careful with this). Jailkeeper and Masons are also probably okay.
I might think of more things to add later, but I just spent way more time on this than I meant to.