Seems like we're all over the country, and honestly, I'd try my damndest to get to wherever PP18 happens to be. But....I'm lucky cuz I'm already middle-ish.
In the spirit of gathering the largest herd of swine, I think location and timing are key to that, like many of ya's already said.
I like the idear of central ish locations. Give or take a few hundreds of miles N,S,E or W.
Summer is most likely easiest for anyone bringing kiddo's who still attend school. Whether they come along or go to "Gam Gam's"....I swear those summer breaks get shorter every year. Early June-Mid August?
Mid summer does make me inclined to think Northerly for temp purposes. Tho, since I spend most of the year neck deep in snow, I could use a good warming up.
Trail options are obviously the high priority.
Soooo....it's only my opinion, but maybe we come up with a decent list. Vote to narrow it down once, cause there's be too many ties (1,2 or 3 votes) then take the winners and have a final vote.