Siopaoo

I agree with what Bash said, it would just make the game feel a bit more tedious than it needs to be. If you really want a certain uniqueness from each server, others' suggestions here of doing server specific events or event pokemon designs exclusive to only one server would be a good start. See how players react to that then go from there if you want to expand the idea. You can have certain mounts or accessories or costumes "native" to the server like a gold and silver uniform or something..