I was thinking they could do it well if it worked like Guild Wars, where if you have GW Prophecies and get Nightfall you can bring your Prophecies characters over into the Nightfall world (IE Pokemon characters into Mario world) BUT that'd require a harddrive....
Otherwise, each world would need to follow it's own play mechanics to remain true to the IP it's for. Pokemon is ideally suited for a MMO, Fire Emblem and Metroid would adapt well too. Mario, Kirby and Zelda? Not so much. Who wants to be a Toad(ette)? A Koopa Trooper maybe... but still. Not much fun in those IPs unless your the key character, or in Zelda's case, it would just be untrue to the IP to have an armies of heroes completing quests (and *gasp* talking...).
I don't think you realize how vastly complex a MMO is to design, balance and maintain. Mixing multiple IPs would just make it overwhelming hard to balance and make it true to each IP. It's a nice idea, and maybe on some limited scale could be done but I can't see it ever happening beyond SSE in Brawl.







