I agree that the chances of new Allied races are exceedingly low right now. BUT if they are going to happen in a hypothetical 8.3.5 (which I heard was actually confirmed by the devs in a Blizzcon interview, but this could be internet scuttlebutt) then the reason the Mogu would join a faction is because of events that occur inside the Ny'alotha raid, which I will not spoil. Meanwhile the Ankoan are already well disposed towards the Alliance so there wouldn't be too much of a leap in them making things formal.
The Tol'vir won't be happening as they have four legs.