well, the big server idea would have to be run by MTA... and that'd be both expensive and complicated, and wouldn't really fit into the current idea of MTA gameplay.
I mean, MMORPGs have to have monthly fees just to pay for the servers.
That, and a large server would be easy to hack into (although this story showing that is just hilarious :http://wired.com/news/games/0,2101,59034,00.html)