When an opponent buys/gains/trashes a specific card or type of card
It would be cool to have a Remodel-type card that reacts to an opponent trashing a card and grabs it out of the trash. I guess it might be frustrating in multi-player if multiple people want to get the same card out of the trash.
Reacting to other players buying a victory card to top-deck it could be cool too.
Anything where card type is relevant could also work with cost instead. For example, "when you gain a card costing $3 or more..." or "When you play a card costing $5 or more...".
Some other ideas (some are a little crazy):
"When you play your nth action this turn..." or "When you have n cards in play..."
"When you have n [actions/buys/coin/cards in hand]..."
"When you gain a copy of a card you have in play..."
"When you gain a [this]..."
"When you play a [this]..."
"When another player plays a [this]..."
"When another player gains a [this]..."
"When another player reveals a Reaction..." (or just any card; this could get weird though)
"At the start of your Clean-up phase..."
"When you would discard a card from play..."
"When you would end your turn..." (I don't know where that one is going, sounds dangerous)
"When you would end your action phase..." or "When you have no actions remaining..."
"When you would end your buy phase..." or "When you have no buys remaining..."
"When you trash a [this]..."
"At the start of your nth turn..." or "At the start of your turn, after you've taken n turns..."