There are a number of genres/subgenres that are under-utilized or not utilized at all in rpgs, despite the fact they would probably work pretty well. Here are a few off the top of my head:
Basically this would cover the whimsical, fantastical, and often violent world of Thimble Theatre (later Popeye) and the Fleischer Popeye cartoon. A lot of fist-fights, fewer guns. This would also cover Little Orphan Annie, various kid gang comics, and (on the more violent end) Dick Tracy.
Little creatures hiding in the big world. Think The Burrowers, The Littles, and Fraggle Rock.
Scooby Doo is probably the most well-known example, but you've got several Hanna-Barbera returns to the same concept. Ditch weird pet/side kick, and you've got The Three Investigators, Nancy Drew, and the Hardy Boys.
I've written about this one before--and Richard has run it. Still needs a game, though.
5 minutes ago