With all the speculation for races etc in future expansions, two that VERY regularly that come up are..
Naga for Alliance and Ogre for Horde.
The problem with both of these is mechanical. Ogres are larger than taurens and will look ridiculous on any mounts, let alone be able to fit through doors.
Naga have tails so can't really straddle many mounts.
Most obvious solution i'd say is design a SPECIFIC ground and flying mount /spell for each race... and then they can't use any other mounts. NONE.
Most players have numerous, if not dozens, of other characters that can use all of their other mounts... but when you have hundreds of mounts, would you be ok having one character that you can't use them on?