There are plenty of planets that would be awsome for an expantion(taloraan!!), but from a marketing perspective, it would be most efficiant to use planets that non-hardcore star wars fans have heard of.
I think naboo would be a great planet because of the fact you could have the underwater gungan citys, along with the countryside, and citys such as theed.
Kashyyyk would be great because wookies are such a great part of the star wars universe.
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.