It could be interesting but a proper one would be so far apart from the series roots and other MMOs on the market. You'd have to get rid of the "gotta catch 'em all" slant because not everyone should have the legendaries. WoW raid style content would not work at all, save a few possible, tenuous exceptions. Because Pokemon doesn't seem suited for mass cooperative activities, the maximum server population would have to be far lower than other MMOs.
I think the proper direction of such a game would be something like a life sim with a heavy emphasis on exploration. However, to make it worthwhile, the environments would have to be rendered in dramatically more detail than anything we've seen so far. I suppose group activities could revolve around gyms, perhaps each operated by something like a guild. I think there's story and gameplay potential with the series' science fiction and archaeological elements. I'm probably just creatively limited but doing this seems so daunting that I wonder if any developer (from anywhere) has the skill or if any publisher has the willpower to do it correctly.