It's a bit of a quandary, because most trashing happens under the player's own control. Making it vulnerable to Saboteur, Knights, and Rogue is a start. Making it a Treasure would make it vulnerable to Thief and Pirate Ship as well, plus Loan (which a player with one of these just wouldn't buy, but if the card makes you avoid Loan, that's still an interaction of sorts). But that's still only a few cards out of many. Otherwise, the player just chooses not to trash the thing, and it's a wash.
So I think the on-trash effect would have to be either situationally beneficial (which is not really in keeping with your idea) or some kind of situationally forced trash. Like this, which is not a real suggestion, just an illustration of what I mean:
Just an Illustration of What I Mean
$3 - Treasure
Reveal the top 4 cards of your deck. This is worth $1 for each differently named card revealed in this way.
If all revealed cards share a type, trash this.
--
When you trash this, gain a Curse.
In other words, the on-trash effect is bad and sometimes it is unavoidable. I think cards using this kind of structure will be inherently swingy, though, and the other thing is that if you're going to go to this kind of trouble, is this really better than simply saying "If all revealed cards share a type, trash this and gain a Curse" and dropping the on-trash entirely?
So even with some convolutions to make the card trash itself sometimes, it's still a mechanic that requires Saboteur, Knights, Rogue, etc, in the kingdom to exploit.