Unfortunately, this one isn't up to us.
In Halo CE, Halo 2, and Halo 3, there were a lot of different groups that had different views on how Halo should be played. While this led to debates, and sometimes shouting matches, there was still a bit of unity there. Why? Because we all loved HCE, H2, and H3. Even if we didn't, we could all say that it was still more fun than most FPS games on the market.
The main split in the community didn't come from casual vs competitive, MLG vs HLG, Canon followers vs game lovers. The main split started when Halo Reach was released and Bungie announced it was leaving Halo. Reach was the first game released that could actually be hated by the different groups within the community. A lot of casual players love Reach (mind you, I say a lot) and a lot of competitive players hate it. MLG and HLG was for the most part unwowed by Reach. Canon followers despised Reach and the game lovers either loved it or hated it.
The full split happened when 343i took over Halo. The community was shocked by how Reach turned out, and then was left with confusion about the company that they had known and come to love (which is rare with a game developer company) being replaced by an unknown and unproven entity. This pretty much shattered the Halo community as we knew it.
The bad news is that there is nothing we can do to fix this. The good news is that it is fixable though. 343i has to prove to the community that they can make Halo great again, and make Halo 4 the best game it can possibly be.
In my own educated and humble opinion, the only thing we can do is try to prevent the forest fire from spreading and pray for the rain to come and end the drought, if you'll forgive the metaphor.