I doubt that the legendaries would be as easily catchable as they are in their handheld incarnations. You'd have to do something really drastic or epic to have a meeting with them.

Somehow, I'm seeing this like the anime show, since there is a basis for somewhat realistic actions. Legendaries wouldn't really be catchable, and if they were, it's an immoral action. Player-established gyms could come from people with enough battle wins and good enough reputation (needs more Listchecker).

Where's RPG maker? Does that multiplayer functionality? We need this NOW.

Either way, it'd have to be pulled off very well. If they started making this, I would buy it. Twice.