When you gain... you may shuffle it into your deck
Yeah, I know this (as written) is a weak watchtower. But depending on the things you add to it, it could be made unique. Like when you gain a victory card, discard this. Gain two silvers. Shuffle the victory and silvers into your deck. Or whatever, it's just to point out that there's design space here, not that it's a fully-fleshed out idea yet.
When you gain... you may trash it and gain two cards each costing less
Yeah, I'm imagining we are going to move to something other than "reveal this." Discard probably best here. Imagined as a sort of on-buy Stonemason effect.
When you gain... you may set it aside and put it in your hand at end of turn (after drawing)
Yeah, somewhat similar to Summon, but without the restriction to actions or the obligatory playing of it. But not really that much like watchtower. There is considerable difference between topdecking and setting aside to add to your hand after the draw. The former can set up a good hand for next turn, but still limits you to five; the latter gives you a better overall hand, though at the cost of vulnerability to handsize reduction attacks--but certainly putting you in no weaker position. It also has a different effect with gainers during your action phase, allowing you to set up a good hand for next turn if you don't need the very same cards this turn. Again, just saying there's possibly interesting space for developing it here.
And again, I voted for "when somebody resolves an action." So I'm more interested in the possibilities of that design space.