Yeah, another race or two would be nice, but each additional race would make race-balancing exponentially more difficult.  (Vasari has been nerfed so much that now you're starting to see fewer and fewer people pick Vasari in the humans v. humans online multiplayer games, essentially making it an Advent and TEC only game.)  It would be nice to see another race or two in a Sins-2 if they could work it out along with the option to have some more planet types (such as the Sins Plus planets).