Tried playing Pokemon Crater once. When there are far too many players on the screen at once, so that you can barely even see the ground and it takes a good several moments to take even one step, you know the game sucks.
Wait for Nintendo to make one.
Come to think of it, how does one make a Pokemon MMORPG?
It sounds like a good idea in practice, but your superiority over just about all the other NPC's was the reason it was possible to progress in the game. However, to beat even three human opponents so as to make it to the next town requires that only a very small percentage of players, in this case between 1 in 4 and 1 in 8, are capable of doing it without fainting. That's assuming an extremely small amount of players per route as well.
Not only that, but such an endeavor would require hundreds of alternate servers and player clipping simply to see the ground and enter and exit Pokemon Centers.
Personally though, I would love to see a Pokemon Mystery Dungeon MMORPG. Almost all the characters would be unique because you could select any pokemon to play as (with Pikachu being the most prevalent probably, followed by Jigglypuff, Lucario, and Mewtwo), a lot of emphasis would be placed on joining explorer teams with other players to counter your weaknesses, and tournaments could be held elimination style - stick everyone in a large room and see who survives. It could also make the mystery dungeon gameplay less dull because you would have companions.
Although turn-based gameplay might have to go.
Actually, forget a Pokemon MMO, I want to play
any MMO Nintendo might come up with. They are really good at making games reguardless of genre and franchise, and the mere fact that 1. it's made by Nintendo and 2. It's an MMO will mean that it will be among the best games ever created.
EDIT: But absolutely no wii-speak. Keyboard is fine, but Halo middle schoolers are annoying and everywhere and never stop swearing. Loudly.