That's a very good mechanic!
I'm going to go with Karma, as used in the old FASERIP Marvel Super Heroes and Earthdawn. So you this resource (in Marvel it was your xp, in Earthdawn it cost xp) that you could use to add more oomph to rolls when you really needed to succeed, but since using it cost xp that you could use to progress, players have a vested interest in being stingy about it, as opposed to using Karma willy nilly. There are times when a campaign can really hinge on whether or not a key roll succeeds or not, and I've always liked giving players a resource to use in case of such emergencies, but usually, they either never use it, because they have no direct way to get more of it, or they spam it like it's going out of style.
By tying it to progression, you have a very simple "you win but not without a cost" mechanic.