Here's an idea, without mucking too much with the basic thief:
what if thief may place one of the revealed cards back on the deck?
That way, you also tack a spy/rabble-ish effect on, which might just make it acceptable.
Or maybe make it mandatory, otherwise it might become too good at $4, when bought mid/late game. Or not. I haven't thought this through very well
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PS: so what I mean: you are still forced to trash coppers but you also get to put a green card on top.
PS2: sorry for reviving this, apparantely I clicked a link in a different thread and ended up in this one, while forgetting about clicking anywhere. Again, sorry.