I like the caveman idea. I've already thought about some kind of prehistoric action-startegy-civilization-god game, controlling little tribes, developing/using simple technologies and weapons, buildijng small settlements... moreover, later it could be adjusted to other eras easily, like in case of many rts games...

the magic shooter is also a fine thing, but probably requires advanced artistic and effect writing skills...

what I plan currently is to make some little open source games, to replace my demos representing how to use levels created in MapBuilder, with some more advanced stuff. they would be basically 3rd person shooters, or sword and bow fighters, or spell casters with some... but maybe I'll be lazy and work only my rts game