Geonisious(sorry for bad spelling) would be excellent because of the arenas. It would be a great basis for some kind of arena style pvp.
Also, yavin 4 because in the knost dural jurnal entries it talks about the dark temples, which could be a great plot basis.
Geonosis I doubt, since it's basically a rocky wasteland and we enough wasteland plantes already (Korriban, Tatooine, Hoth, ...). Yavin IV would be pretty cool.
Personally I'd like to revisit Manaan too. Koltoi is referenced several times in the game and it would make sense for both sides to fight over control of it. Dantooine runs a close second.