It has been so rare in the past that I've been a player and not the GM that I sometimes have a hard time coming up with characters concepts that I'd enjoy playing for an extended time, rather than the one-off or occasionally-recurring characters that a GM usually needs to make. So, that's the only reason this idea is notable at all, and I wanted to write it down somewhere so that it wouldn't pass into my mental ether before I ever get a chance to use it.
A while ago I read with interest Jim Henley's Actual Play of a game with his kids set in the DC Animated Universe, which planted the seed. Greywulf has been on a huge Supers kick for the past while, throwing up fully-statted and CG-illustrated characters at an impressive rate. He got me interested enough in Mutants and Masterminds to read this great review of M&M 2nd Edition at RPGnet by Jacob X that sold me on the entire genre. (I've never been a Supers fan, but I like what I read there.)
So today Grief jumped into my brain and made the final connection between thinking about Supers roleplaying and actually wanting to play.
Grief is a tragic hero, maybe an anti-hero. His power is to make anyone relive their moment of greatest grief and pain magnified a hundredfold. It disables his opponents, stopping guards and thugs in their tracks, and (so he thinks) justly punishes criminals with the vulnerability that they exploit in others. The catch is that he experiences whatever memory he induces, too. He's become somewhat inured to it and isn't disabled by it anymore, but at what cost to his humanity?
Obviously that concept would work best in a noir or Punisher-style game where tortured anti-heroes are interesting. I haven't thought up much of his background, but there's scads of tormenting possibilities with an empath/telepath, let alone one who can also project and magnify what he perceives.
A variation on Grief I considered after the initial idea was the Rag Man, whose power also burns the memory from their mind as he induces and intensifies it to paralytic proportions. Lifting these horrible memories from their minds like an old rag and smoothing it over like a fresh linen, though, he permanently etches them into his own memory, accumulating a debt of grief and sorrow that might yet bury him. And what of those who realise what he's done and, despite the sadness of the memory, treasured it? His arch-nemesis might be only a relentless cop who knows his only memory of his dead daughter has been taken from him and who now has nothing the Rag Man can use against him. (A cookie for anyone who gets the reference in the name.)
I don't have a system in mind. Oddly, I think either might work in a game of Sorcerer, given that sort of humanity-testing internal conflict.