I have an idea: Why not go the Mass Effect 3 route.
Two types of DLC, multiplayer, and story. Story DLC allows us to play new chapters of Campaign or Spartan Ops. Not to fill gaps between the beginning and end like Mass Effect’s does, but to add to the story with mini-campaigns following different characters, or possibly telling a bit of Master Chief’s story from in-between Halo 4 and 5: Guardians.
Story DLC will need to be bought. It may come complete with new Armour, Weapon Skins, and some other customization options for multiplayer, but nothing new that you require to play the game. It will also be the only DLC to include new achievements for the game. (If the DLC is scheduled to alternate, or be released side by side with one Multiplayer expansion, and one Campaign one, new Multiplayer Achievements for the maps will be included in either the following, or the released alongside Story DLC.)
Multiplayer DLC on the other hand is where we get our new Maps. And that comes free.
If you are going to have paid Multiplayer DLC, don’t say “Hey DLC guys, here’s a corner of the game you can play in.” Instead say “Hey, non-DLC guys, here’s a corner of the game you can play in.”
Put DLC requirements on playlists that make sense to have it. And leave 3-4 basic experience playlists for non-DLC owners. For instance, Team Slayer shouldn’t require DLC, but Big Team should (because BTB maps won’t play anywhere EXCEPT BTB). Rumble Pit shouldn’t require DLC, but SWAT should, MLG/Throwdown/Proving Grounds should also require DLC provided any maps that competitive communities use are DLC maps.
In other words AT LEAST HALF the playlists should require DLC. Those willing to subscribe to the continuing Halo experience deserve to play it. Those that don’t should still get to play Halo’s multiplayer, and the core game it’s known for (6P FFA, 4v4 Slayer/Objective), but the niche experience that isn’t basic to Halo aught to be reserved for DLC buyers who continue to support the game post-release.