I like the idea a lot, but I'm worried that with different players receiving different Trinkets, there can easily be an unfair advantage for one player over another; depending completely on what else is on the board.
Rare Coin
Treasure/Trinket - $4
Worth $1
As already mentioned, sometimes 100% the same as Copper, sometimes really good.
Toy Bridge (credit to dondon151)
Treasure/Trinket - $0
Worth $0
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While this is in play, cards cost $1 less, to a minimum cost of $0
Same as above... though it's more common to have at least some option of +buy. I like this one, basically a Silver if you buy 2 cards.
Key to the City
Treasure/Trinket - $0
Worth $1
+$1 for each empty supply pile
Needs "when you play this" type wording. The bonus isn't nearly enough to make a player try to empty a pile if they wouldn't otherwise, but it still works out just fine if you keep it to the end of the game.
Bell
Action/Trinket - $0
+2 Actions
+$1
Probably the most powerful Trinket. Interesting that it's an Action and not a Treasure. But completely unfair if it's a board that would be great for an engine except there's no Villages; the player who gets this probably automatically wins. Especially if there's something like TR or KC.
Dagger
Treasure/Reaction/Trinket - $0
Worth $1
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When another player plays an Attack, you may trash this. If you do, you are unaffected by that Attack.
Needs "from hand" wording. I would like it better if you discarded it instead of trashed it. Trashing is weird because it can be really good (free Copper trash), but maybe you don't want to trash it because it's stopping attacks.
Locket
Treasure/Victory/Trinket - $0
Worth 1VP for every Trinket in your deck
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Worth $1
I agree with above; should just be a flat 1 VP. Also the only Trinket which has some sort of use in every game.