I like the general idea. One question is, how many Turncoats are there, and does it very with the number of players? I don't think it's worth having 50 Turncoats just for this attack.
I have to ask myself if the end result here is really "worth it". Basically it's a discard attack that's more likely to hit the more of them you (and other players) buy. On the face of it, that seems weaker than Militia. It's potentially more devastating if a player starts with several Turncoats in hand. And being forced to essentially discard a random card hurts more than discarding your weakest card. Hmm. Maybe $5 is a better price. And maybe it's too harsh in general.
I think I'd rather see Cabal actually discard Turncoats rather than topdecking them. Turncoat's on-trash effect is still valid for other purposes, but discarding both makes Cabal cleaner and makes it harder to lock other players out.
Back in the day I had an idea for a fan set that used a penaltry card similar to Turncoat.
Cave-In: Cave-In, $0*
When you trash this, put it on top of your deck.
(This is not in the Supply.)
The idea was that you'd buy a powerful Action card and then get a Cave-In with it as a sort of limiter. And some of the Action cards would interact with Cave-Ins, etc. Anyway this was all over two years ago and you can read about it here:
http://forum.dominionstrategy.com/index.php?topic=5566.0Originally I was going to pursue that after finishing Enterprise (if that ever happens). Later I decided that having a whole expansion mechanic based on every card having a penalty sucked. For the most part, penalties aren't fun. But! If the self-inflicted "penalty" card were a cantrip like Turncoat, the idea might work! Some of the cards wouldn't work and the rest would have to be rebalanced. It probably wouldn't make a good mechanic for an
entire set, but could easily produce a bunch of cool cards.