Being the most densely populated area, the East Coast of the US works the best for huge meets. I'm one of the organizers of the largest gathering of Ford Probe and Mazda MX-6 owners every year. We take over Myrtle Beach, SC for a weekend in November every year. Last November, we had over 300 people attend from as far across the country as California and Washington and even had a guy take military leave and fly in from ITALY.
HUGE meets are great fun, but they take a LOT of planning. When you're getting people from that far, you have to have a good plan and real substance.
My opinion? What better place than the National Aquarium in Baltimore. It's relatively central in the east coast and, well, it's the National Aquarium. There's plenty to do in the area.
I'd be more than willing to assist in planning the event. We really just need block hotel reservations for a weekend and let some key restaurants know we're coming and make plans. If anyone has a better idea, let's hear it. But seriously, we're not gonna drive hours and hours to hang out at someone's house for a few hours. Sure, it's the first thing that comes to mind, but in reality, it doesn't work out for a large-scale meet.
Having said that... If this gets going and there are people there, I'll attend almost anywhere along the east coast.