Well, It would be great if they brought it back to online play, but this would be extremely difficult as it was Microsoft that shut down all original xbox game multiplayer features, not Bungie. And on an extra side note most people would find it difficult when they can't chat to their friends or whoever else while playing it online, as it is an original xbox game, and all xbox 360 online features don't work while playing an original xbox game.
However, I think that Bungie should update Halo 2 (graphics, multiplayer, gameplay, ect.) and release it for Games on Demand like they did with Halo CE. This would be much easier than getting Microsoft to waste their money on activating a whole new online server, and it would work better too.