No way. All playable toons should be biped. Imagine how silly a mammoth character would look on any mount?
At the moment it can be very annoying when those annoying druids with big bear asses sit in you fish heast and you can't reach it. Hell imagine these suckers blocking everything. No thanks.
If I had to pick a new playable race it would have to be something like Ethereals. And perhaps make that the first race that has to decide at the end of the starting area which side they choose Horde or Alliance. Would be fun to have a playable race that can be in either faction. Changing should not be possible then besides using the fee-based transfer like any other faction switch.
Abilities based on displacing stuff and manipulation at a molecular level.