Mass-emailing; I don't think that would work, as Mike said probably most people have different email addresses and sending an email to 50k members would be very resource tasking on the server, with a very unlikely chance of success. Reorganising could be a good idea, but we'd have to let everyone know somehow, and, as said, internet culture is changing and forums aren't what they used to be. GTAForums is bigger because it was one of the first, and it always had a lot of activity, don't take me wrong on this though, not trying to compare anything.
Reviving the multiplayer scene would be a good idea, problem is, hackers would probably come back and ruin the party, and there still would have to be some form of contact in-between missing members to let them know. Contests are definitely a good idea, Facebook and Twitter spreads are always good to let people know. Maybe we should use Twitter/Facebook more often to report news and let people know? I'm clueless, though, most posts already got the idea