What about the Denobulan? Someone will probably want to play a Dr. Phlox with their new ENT uniform.
I had for a bit a medical officer that was part Denobulan and part Trill, one of the few times I referenced a canon character with one of my characters, since Denobulans were very large families with multiple wives and husbands. I felt more confortable stating that my character was distantly related to Phlox
The way the whole overall story line is playing out, I think it's only a matter of time before we see Federation playable Cardassians and Romulans. The Federation has an agreement with Cardassia to provide security for Cardissia and it will in exchange disband it's military. It is only logical that there will be Cardassians joining Starfleet with the Federation's new policy allowing for dual citizenship, and there were several Romulan colonies that asked for Federation protection during the strife that followed the destruction of Romulus.
I would actually like to see the Grigari make an appearance somehow. Not as a playable race though; maybe as the subject of a weekly series or something. They are essentially a deus ex machina species, true, but they're deeply creepy nonetheless.
Of course, the main problem would be convincing the Reeves-Stevens to give Cryptic permission to use them. Well, that and figuring out how to animate them. Maybe once the Tholians are done they might get a look in...