Races that are mentioned in the game and may exist.

I voted for the Nagas because this race should have been in the third expansion pack. The third expansion pack wasn't made, but some of the development work can be found in the resource files of Tribes of the East, which is what comrade Psatkha did:
http://narod.yandex.ru/100.xhtml?psatkha.narod.ru/NCF_Others/test_30.rar

Draw your own conclusions.