If halo pc was online co-op, it would be awesome! And imagine if you could race against eachother!
True, you have to build maps Designed to have multiplayer. And you would have an add-on to make it work. Halo it self would not let you just join in, and, it would have to be an instance (Like the Portals in WoW) And then no one could join the game after you begin playing.
Halo: CE lags too much anyway.
It lags just as much as the regular PC version, what are you talking about? All you have to do is find a server with low ping and lag won't be a problem.