The host city will have to have a large local contingent to pull off a third day (IMO at least). For those of us that do LD travel for these meets it's a constant battle between time off and time at the meet and I for one know that selling my wife on the idea of being gone for 4 to 5 days at a headphone meet is NEVER going to fly unless the meet was in Boston.
And FWIW I don't happen to think that Boston could pull it off without a major overhaul of the local community. Shit, we haven't pulled off a local meet in almost two years.