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player interaction (especially non-attack player interaction) is definitely a highlight goal for me with these (esp. in the third set, "Syndicates", but that's future knowledge). The supply pile trashing is a minor theme, nothing overboard.
A general disclaimanifesto: a pervasive subtheme in many of my designs (both in dominion and in other/original board game designs) is hubris cards/abilities - higher risk cards or strategies that generally don't pay off. While this is boring to a competitive scene (there's usually a more optimal/dependable path), it enables the "big fish" stories that more casual game scenes thrive off of. This is the same design space that MtG's "Un-" sets live in; it sees cards like Coppersmith, Tribute, or snagging three coppers per turn for a Thief-Garden combo as worthwhile paths to examine, not missteps. I'd encourage looking at your own designs from this perspective. Ask whether a thing is fun when it pays off rarely - is the payoff story worth the times it doesnt work? Is the randomness fun for a new player? Since most gamers play a game only a handful of times, I'd say light constant randomness (shuffling) and heavy intermittent randomness (rare payoff strategies) are both crucial to a design to encourge a positive memory of a game, despite ths latter discouraging data-driven replayability.
Yes, this is a fine way to expand the game. Those who like a more strategic Dominion can always stick to Donald's cards, can't they?
re: what you're buying with Prohibition, you're adding a token delivery mechanic to a pile (similar to a conceptual Adventure token for +1 Coffers) but it affects all players (a la Embargo). What the prohibition tokens do is resolve when the Vice Raid and Valuable Contraband Causes come up, as well as provide a future design space thematically based around bootlegging/smuggling.
So 5 x players tokens go on the event, ready for taking. Are there going to be other ways to get them besides the Prohibited State? If there aren't then the number of tokens won't match up to the number of cards in the Prohibited pile in 3+ player games.
And what I meant with buying was, you pay $4 to use the Event, what happens?
The mixed piles will be expanded upon in my second set, "Heresy"; for now they're largely cards that are variations on a theme, synergize in a way that doesn't necessitate one happening first a la split piles, and/or a collection of lower cost cards that would maybe be boring to have all three as separate kingdom cards but put them all together in one pile and you get interesting interactions. Having a nonconsistent supply pile can create interesting randomness in the (un)availability of the card you want.
This could well work, and with your intent on the player interaction side it looks like a good mechanic to explore. It might just be Slaughterhouse/Cattleman I don't really like - terminal Actions are the ones players want to get right in their decks, and these two behave quite differently. So, I could see a mixed pile of very similar and/or non-terminal cards working better.
At least, this is from a strategy player's point of view. I'll try to be fair looking at the individual cards:
Stampede - I can't help but point out that Donald found a top-of-deck trash Attack did not work, and he really tried to make it happen! The exeunt of Saboteur from 2E Intrigue tells the whole story. Someone has to trash a Province, another a Copper or something not serious. By pure chance the player with the Province is losing. There's also the potential to reduce players' decks to nothing and render them helpless. I guess this really establishes the audience you're after, some people will like the 'gotcha' moments but strategy gamers will be frustrated at this.
Witchhunt - the buyer hopes to get a Silver on the deck, at a risk that's more favourable the more players there are. I doubt this costs $5, maybe $4 or even $3. Besides this it captures the casual spirit of Stampede in a less volatile way, so it's good.
Abbey - a purely strategic one. I first wonder if someone would want to pay $4 for this, but Island-ing a Province is valuable. And too cheap might speed Miser up too much or make readying Reserves too easy.
Fallow - trashing a Treasure is optional as you can always just play all your Treasures down before buying this, but Copper trashing will often be what this is used for. So effectively it costs $4 to trash one Copper, so much weaker than Bonfire. The kingdom pile trash is situational; aside from cards that gain things from the trash and split/mixed piles, will someone have a spare $3 to go towards slowly emptying piles? So overall, this could be a touch cheaper at maybe $2?
Cow - how many of these are there? The premise looks good, storing these up to spike certain effects.
Rustle/Wanted - Wanted shouldn't be too hard to remember as you take it yourself. It can add reason to why you get Rustle, pointing towards either a big money strategy or a trash-Actions-for-benefit one. Rustle itself can essentially be a Silver in Action form - you get 2 Cows to hand and reserve them, never calling them into deck again. This suggests a higher price of $3 or $4. Of course it depends on how many Cows there are, once they empty out you're taking them from others and it becomes a Copper.
If other Cattle cards are in the game this becomes a bit more potent and more fun, but usable without if an all random draft makes such happen.
Milkmaid - compared to Miser: in exchange for having the desired Treasure from the start and thinning the deck, reserving them is non-terminal and you get a buy. Difficult to tell without playtesting, but the buildup speed feels about the same. You can change the number of cows that come with it if needs be.
Slaughterhouse/Cattleman - to elaborate on what I said earlier, Slaughterhouse trashes the cows, whereas Cattleman can reuse them later. It seems a lot more worthwhile to make a Cattleman deck, one that can handle the cows coming and going; you need only draw a Copper's worth each time to achieve the same amount of payload as Slaughterhouse, albeit without the pseudo buy. It's more flexible too, Slaughterhouse emphasises 'store 4 cows for a Province' quite strongly but Cattleman could say cycle 2 cows each turn. So, the chance element of the pile feels a bit too harsh here to be liked.
Causes
Valuable Contraband - players want to either time when they can best get a Gold and Duchy, or compete for prohibition tokens first. It feels a fair bit more fun and meaningful with a Prohibited pile, although if this ended up just one card down the Prohibited pile itself...whoever gets to gain that top card first takes a nice prize, yet it's a balanced one not overly dominating early. It should work.
Vice Raid - the Prohibited pile is discouraged...or not, depending on where this ends up. It can be nice to analyse, and I guess the flavour of going innocent or criminal is a nice touch.
A Time to Sow - a simple Cause that quietens the noise of the others, which is good so there isn't too much going on to take in. That said, could a bump up to 2 cards be more meaningful?
I really do feel the Causes are the best thing here, because I suppose they appeal to both casual and serious players!