For a third expansion, you need to think of possible themes for it. Civilizations in expansions almost always fit the theme of the expansion. I could think of two possible ideas, Civilization V: Discovery and Enlightenment (focuses on colonization, vassalage, revolutions, adds a transition from Renaissance and Industrial, etc) and Civilization V: One Small Step (focuses on space exploration, expands the Future era, global warming, MAD, Cold Wars, etc).
Though, at any rate, I feel this list could fit both themes (except the last two couldn't fit my OSS idea).
1) (Gran) Columbia
2) Canada
3) Australia
4) Argentina
5) Mexico
6) Kongo
7) Vietnam
8) Cherokee
9) Holy Rome
Eh, even if you weren't suppose to, everyone makes mistakes once and a while.