DATES: So far, it seems like the best time is August 2016. (July doesn't work for Sharecrow, and September doesn't work for Kreegs.) Might not work for everyone.
LOCATIONS that have been put forward so far are Toronto, Chicago, Las Vegas, and Minneapolis. I whipped up a quick and dirty estimate of flight costs from various cities to these locations just using Expedia and picking the 2nd or 3rd cheapest option (trying to ignore outliers):
These are obviously very rough and very subject to change. Nothing too surprising here. Las Vegas works for the west, and Chicago looks generally best for the east. (On a whim, I checked flights from Hamburg, and their cheapest option is to Toronto, around $1500-1900.)
Based on these results, I'm going to suggest removing Toronto and Minneapolis from the location pool. The people most affected by this would be Shoe (prefers to stay in Canada—Vegas and Chicago will both only cost you an extra $90), Han (lives in Toronto area so it would be supercheap), and Kreegs (lives in Minneapolis so it would be supercheap—at least Minneapolis seems to have cheap flights to everywhere else!).
We'll worry about accommodations later on.
Totally the most convenient location. Only a short flight for KawaiiPikachu and then like a 24 hour flight for everyone else 
