www.LatinaButtFuck.com Latina + Anal
www.TeenSlutBox.com Teen + School Girl
www.BestBabeVideos.com Babes + Teens + Big Boobs
There what I've started out with. LBF and TSB are literally still being made and so havn't made nothing. BBV isn't doing too great on sales, but it's also still new (April Launch) but has made advertising money.
I'd say think of something VERY specific, and then be VERY specific.
Good luck