Why the hell not?
It wouldn't be conceptually complex to make, Pokemon is a relatively simply RPG after all. I could picture it now: a massive open world, lots of trainers with Pokemon at their side, Rattatas running in the tall grass, Pidgeys flying overhead. Big neon cities where trainers come to show off their pokemans and do battle. Combat would be kept turn-based of course.
The idea has loads of potential, but obviously a Pokemon MMO would never happen because it's Nintendo we're talking about.