Hum... I think we should keep it simple. Spyro and Crash are the only two franchises that have this weird connection for being the unofficial PlayStation animal mascots during the PS1 days, and that's why they fit together in this game. It's also very convenient that Activision owns the rights for both of them.
If they did what you ask for, pretty soon people would start asking for Sir Daniel Fortesque (Medievil), the clown from Twisted Metal, Spike (Ape Escape) and Lara Croft. Where does it stop? haha
I say keep it within the Crash universe this time, with Spyro as a guest. If one day someone decides to make a PlayStation All-Star Racing, sure, the sky is the limit, but that's a whole different story :P