I've recently fell in love with Mutants & Masterminds 2nd Edition. Finally an intuitive way to use the d20 System without all the complications that makes d20... annoying.
So far, I have two ideas as to what to host. One is quite simple, the other is far more sinister and dark.
The first idea is that based off of City of Heroes. Basically, the players are from Paragon City (or elsewhere) work for the various H.E.R.O. corps and such and defeat badguys in a mixture of both Iron Age and Silver Age methods. (Defeat some badguys, kill the aliens, monsters and such.)
The second idea revolves around the traditional SMT line. Those unfamiliar with it, will find some sort of catastrophe is about to happen or happened and the "protagonists" are granted unusual abilities (with a twist always to them) to battle such. Quite mature in storyline, Iron Age material. You never defeat the bad guy, you kill him, maim him and in some cases (for some types) eat him. It'd follow a more dark aspect then the one above.
Now there you go, two ideas. Both using only the core for Mutants & Masterminds 2nd Ed (seeing as that's all I own at the moment).
Both are dark in their own retrospects. But the first one is a mixture of light and dark, while the second is more occult gothic horror.
In honesty, I'd love to do both, but I need to know what people would be willing to play first.