Heh, then you'd literally have the Zerg...
Though there's an obvious balance issue in a MMOFPS for Starcraft...
Terrans are numerous but fairly weak individually.
Zerg are extremely numerous and extremely weak, individually.
Protos are few but strong, individually.
In an FPS, individual strength matters as a good team balance is not guaranteed; you'd want maybe 1 Protos to 2 Terrans to 3 Zerg or something similar. How'd they manage that?
Unless, of course, the only playable race was Terrans... hmm...