How about "When any player gains an India..." as the trigger?
I still don't think the reaction should work for gained Action cards, even specifically India. India's on-play effect already does stuff when it gains an Action card, so having the reaction overlap like that will likely cause timing confusion. I brought this point up early and nobody's really given an argument for the other side. Do you think it would be OK anyway?
Fair enough. Could it be emended to 'This cannot cause you to play more than one Action', like the wording of Mission?
I don't think the wording of Mission works well here. Mission says "you can't buy cards", which doesn't really conflict with anything. But what happens when I use this reaction to play Golem? I dig for 2 action cards... then I can't play them? Golem requires that I play them, which is actively contradicting the restriction from the reaction. If we assume that the restriction takes precedence (which is
not currently part of Dominion rules), what happens next? Do I put the revealed Action cards back on top of my deck, or discard them? It's not defined.
The most similar case I can think of with Mission is Black Market, and I'm not sure what happens there. But Black Market doesn't require that you buy a card, so it's not quite the same.
Or could it be, instead, 'When a player gains an Action, you may play this from your hand?'
Again, the top already does stuff with a gained Action. "You may play this" just makes this into a reaction that gains, which also causes messy nesting and timing issues. Caravan Guard gets away with it because it's so simple and playing it out of turn won't chain into more effects or trigger more reactions, as would be the case with this card, which can trigger when-trash and when-gain effects and allow various players to trigger more reactions to said trashes and gains.
I'll just quote myself...
Keep in mind that the top already does something to gained action cards. Maybe the reaction should be conditional to Treasure cards to avoid overlap and confusing timing. Victory wouldn't work since Action-Victory is a thing.
Also keep in mind that simplicity remains important or the text may not fit on the card.
I don't like the idea of a reaction that gains because then you have to be super careful about turn order with 3+ players. There will be cases where you need to know if another player is going to reveal before you do, and bad nesting interactions where you react to the reaction reacting to another reaction and so on.
I'm interested in hearing counterarguments though.
Sorry for being so negative. The top is already complex and powerful enough that I'm not sure
any reaction will make a good addition to it. IMO, none of the ideas discussed so far (including my own) are particularly compelling or a good fit. I'm just hoping that the winning reaction won't cause turns to knot up with players interrupting one another repeatedly to do too much stuff outside of their own turn.