As far as setting, plot and characters are concerned, I will leave it in your hands. You're more than capable with it. I do think, however, that it should be an original IP, original setting. It doesn't have to be something entirely new, just something original with a certain degree of complexity. (Plus you get to milk the IP forever afterwards, since it's going to be entirely yours.)
As far as gameplay is concerned, I don't mind whatever combat system it has as long as it's reasonably balance, you can't gimp a character without some effort and, for the love of god, not twitch-based. I do