Welcome to the forums! You should brush up on all the
other existing cards -- it's fascinating stuff. Also check out
thoughts from Donald X (creator of Dominion) on the design of all the different sets.
And if you're up for it, you might also want to read through this
comprehensive guide on fan card creation.
Thoughts on these cards:
Mob -- Gaining a Curse is a really steep price to pay. Too steep to risk this card, I think. The low price doesn't justify the drawback from a regular Village.
Demagogue -- It's OK, but is perhaps too similar to Forge.
Executioner -- What happens if you don't have an Action card in your hand? According to Dominion rules, you continue on with the rest of the card and still gain a Gold. Therefore, this card is too powerful. Also note that your wording has accountability issues -- I could say I have no such cards in my hand and you have no choice but to take my word for it. Official cards usually deal with this by either having the card player reveal their hand (to prove that it doesn't contain the card in question) or by adding an "if you do" clause to the subsequent benefit.
Slave Market -- Since it is one-shot, I think this might be OK.
Ruffian -- Official Dominion is designed not to be political. This card would be rather political (do I switch with Alice, or with Bob?). If you like some politics in your game, though, by all means go for it. I would personally be against a card like this simply because it is so, so, so swingy.
City Watch -- There is no "Defence" type. Perhaps you mean Reaction? For your "until the start of your next turn" wording, you should review the Seaside expansion, particularly Lighthouse. One other issue with this card is that it is almost entirely useless if there is no Attack on the board. Note that other reactions (from your experience, Moat and Secret Chamber) still have some function even in the absence of Attacks.
Visionary -- As worded, this card does nothing but end your turn. You can't hold onto your hand between turns; you discard and draw a new hand of 5!
Garderobe -- Wording should read, "Choose one: trash a card from your hand; or, +1 Card, discard 2 cards." I think this is alright. Might pick it up when I really want that slow trashing (heavy Cursing games). The second option is really bad though.
Chamberlain -- Maybe too similar to Herbalist. Might be interesting as a Treasure type, but I think it's too weak as a terminal action.
Architect -- Wording should say "Victory cards". This is strictly inferior to Cellar.
Burgomaster -- This is too strong. Gaining Silver to hand means it basically has an additional +$2. That makes it a terminal Gold, which is considered too strong for $5. Also note that this card should be an Attack type, since it forces opponents to gain Copper. Yep, that's bad for them! Donald X. has also stated that Copper-giving attacks do not work out in practice due to the way the piles scale in size with the number of players. Existing Copper givers (e.g. Jester, Mountebank) are limited by design, so they work, but unlimited Copper-givers would need careful evaluation and testing, moreso than most of us can put in.
Pickpocket -- Accountability issues again (what if they say they don't have a Treasure card). This is also often better than Thief because it can cripple opponent's hands. "Give you a card" is not well defined -- does that mean you gain the Treasure card, or is it supposed to be like Masquerade and its "passing" mechanic? These are important distinctions.
Fountain -- Probably OK. Terminal Silver is alright at $2. The extra benefit here is super situational (and nearly useless on boards without Cursers -- edge case is you could just buy a Curse yourself). But overall it's probably OK.
Diplomat -- Interesting concept, but overall very weak. Since your opponent can refuse your peace offering, this Reaction only gives opponents more options. The funny thing, however, is that Dominion rules allow you to reveal Reactions as many times as you want. Therefore, if your opponent refuses, you are legally allowed to reveal Diplomat again and offer the same care. Repeatedly. Until they give in out of frustration and despair.
Just like a real Diplomat.
Hope these comments help you as you discover more of Dominion! It's always fun to see more people posting their ideas. You might be interested in checking out the
set design contest. Cheers!