Can you do Characters from Fairy Tail? Like the main four; Natsu, Lucy, Erza and Grey.

Monster High?

Classic movie fanart perhaps? Terminator, Gone with the wind, that jazz

Maledonna! That series really needs some new content UvU
As for fan art, I'm thinking ATLA and LOK, SPN, Doctor Who, and Pokemon!
And I think I remember reading somewhere that you CAN sell Homestuck art if you get permission from Hussie. But if not, you could always make chibi stickers or bookmarks and give them away with orders over certain amounts.

Fanart Avatar the last Airbender! :3 (OH GOD NOT THE MOVIE NOT THE MOVIE. PLEASE THE SHOW)

You should do a little chibi set of mlp as humans! A marceline-PB print or a Prince gumball-marshall lee labulabu pic. More Sailor mooon cuz the new show is coming out and maybe a sexy Rei,Asuka,Mari pic from the new movies?

Have you looked at any of the Korean comics? Noblesse is really popular and I'm sure there would be interested people.