So I've been thinking about this game for some time now but haven't taken any time to write any of this down. I'm thinking of a heist/action rpg game where you develop your own supervillain from the ground up. The game may start off as small as robbing convenience stores, all the way up to world domination. The game will send the appropriate authorities at you, relevant to what sort of heist you're doing. I'd like to have randomly generated superheros trying to take you down along the way, along with one particular superhero that will develop at the same rate as your villain, and he will be your arch nemesis. I'm thinking of having the arch nemesis having skills that purposefully counter your own to make those battles particularly difficult.
I haven't decided what sort of perspective it should play from. A sidescrolling Streets of Rage style, sidescrolling Terraria style, or overhead Gauntlet style all come up as potential candidates here. XCOM style gameplay was also suggested, but I haven't played it so I wouldn't know how relevant that would be. I do know that trying to jump into 3D development would almost certainly be over my head.
As far as skill development goes, I'll probably have a skill tree with points to put in a variety of skills. You use money from your heist jobs to level up your skills.
I think the first step is to get a prototype running and see which playstyle feels the most fun, and then really start to flesh out some of the other core gameplay mechanics.