You either die a hero, or you live long enough to see yourself become the villain.
Two-Face in Batman. The Incredible Hulk. Jekyll and Hyde. Green Goblin.
I'm looking to build a game revolving around a theme where the characters' main focus is to avoid turning into the villain. The heroism bit like fighting villains and rescuing the world would act as background tasks. Most of the player focus, character builds, resource management, and plot would revolve around keeping the character from sliding into villany.
Lovecraftian themed games somewhat fit - the concept that having more knowledge/power increases the likelihood of crippling insanity.
For my specific situation, I'm actually building a gamified task list app. So a party of maybe 4 heroes work together. When the player does tasks, they rescue themselves a little bit. But when someone slacks off, they morph into a villain and the rest of the team has to put in a little extra work to destroy or rescue them.
The theme itself doesn't matter (e.g. fantasy, scifi, horror), but the gameplay would mostly focus around survival and teamwork.