I agree with darkinferno - multiplayer games have the greatest potential if it comes to commercial indie games...
So if you do want to sell something, go for multiplayer...

I'd say lets keep it simple, no next gen fps or something of course, maybe an isometric/third person game, that would make content creation easier as less details are needed.
I'd vote for some rpg, maybe Diablo style? Diablo was hell fun in mp.
And its simple, resources can be created rather easily, several styles are possible... Steampunk, classic fantasy, hell knows.