I wonder if an "outlandish ideas" thread that moderators could direct people to for this kind of thing might be better...
OT: No, generally speaking. If it was an end-of-game, lesser-dragon-surrenders-and-becomes-companion thing done with class and restraint, it could be interesting. But considering the resource requirements (anims, and the rest) I doubt it'd be worth it.
Monster mounts, on the other hand, I'd absolutely love to see in the game (wyverns, giant spiders, unicorns, wolves, bears, elk).