Oh, boy! This won't be easy!
Okay! Those places that only have one MR member, forget it. That's too much work for one person. The host city must have a team ready, one that has the time to set things up.
Nomesque has hit on something interesting. We could have a simultaneous meetup from different points in the world via webcams, but that would be for a short time. And that's not quite the way I'd like to meet you all. I want to look you in the eye while I shake your hand, I want to feel who you are, what your life is about, feel why you are MRers. That can only be done through different points in a series of days.
When I met pshrynk, I felt that we barely scratched the surface.We could have gone on for hours. That is the kind of meeting I would want with you guys.
Since everyone seems to want the meeting in their backyards, we'll have to limit choices to big Metropolis centers. Places already set for conferences and filled with cultural activities. Vegas is out of the question but LA, New York would be OK for that side of the pond. (Montreal is too small) Then London (my favorite choice) Paris, ?Rome?, ?Berlin?, those could qualify, and any other big cities.
If the meetup lasts 3 days, the city would have to provide entertainment enough for a whole week of stay. I would love this to also be a vacation and spend the rest of the week there without travelling out.