Hi, I don't know whether this has already come up here but I think this is an interesting general topic on card design. Self-Topdecking is a bit like Prince, just with a condition.
The official cards that may put themselves on top of the player's deck are Treasury, Alchemist, and Walled Village. So there is already one cantrip self-topdecking card for each of the simple Dominion atoms +$1, +1 Card, and +1 Action, paired with the conditions "buy no victory cards", "play a Potion", "no more than 2 Actions in play".
Are there any other effects that may be worth self-topdecking?
Which conditions make such effects more interesting/ even just possible?
Some ideas:
Religious foundation (Action) $2
+1 Card
+1 Action
Trash a card from your hand.
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When you discard this from play, if your deck is not empty, you may put this on top of your deck.
Here the atom is "trash a card", so this is a Chapel variant. The trashing is not as quick, but more consistent. The condition "if your deck is not empty" is because this can happen easily with a thin deck.
Poet (Action) $3
+1 Action
+1 VP
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When you discard this from play, if you gained a Victory card this turn, you may put this on top of your deck.
This is my first approach to a non-cantrip self-topdecker. You play with 1 less card every turn, but you get VP for it. The condition is the opposite of Treasury because otherwise the game might never end.
Night watchman (Action) $5
+3 Cards
If this is the first card you play this turn: +1 Action.
Otherwise, put this on top of your deck.
What about on-play topdecking? The turn starts with 7 cards if you get to play this in the morning. Otherwise the watchman is kind of sad and waits for the next day. It may be a problem that you could play the same card multiple times in a turn, but I didn't find any issues with that yet.