Heh I think there is a reason why we don't have Phoenixs in the game and why the Gryphons were never implanted even though they did plan them a bit. They just wouldn't work. Simple as that. They are either two powerful and unbalance with the other races or would just be exactly like the dragon which would make them pointless (except for those who prefer them over dragons of course).
You could always go with the "mount" idea. Though I rather see dragon players having this ability instead (something different than what other games have. Have other players be mounts! . But that is another topic for another time.)
There is one more option though that may work. I mean we all agree they are overpowerd right? Then you know what that means! Make them monsters! Who wants to head into the lava pit and mass kill some Phoenixs? Or maybe head to the icy top mountains and blow some Gyphons out of the sky! Would work nicely with those people wanting to see ariel combat for dragons (What? A dragon can't fly and breath fire at the same time?! That's like saying I can't walk and breath air at the same time XD. Again, that is a different conversation for another time.^^)
At this point Phoenixs and Gryphons are just things that would LOOK cool..... but not really be worth time to putting ingame do to various problems with both. (maybe they can make them into shoulderpets.... shoulderpets look cool too *grins*)