(TL:DR AT BOTTOM)
So I think either Frankie or Kiki said that everything you do in multiplayer will have purpose and actually affect things.
But how big are we talking now? Little things like say you complete a difficult task and in the lobbies, your name and accomplishment would be shouted out to all players (which would be cool), or something so insane that it would have repercussions throughout the entire trilogy?
Think about it, what if (assuming we are assigned one side to the Red Vs Blue conflict for the entire trilogy), EVERY kill you get is added up to a global kill count, and at the absolute end of the Reclaimer trilogy when the multiplayer servers are pulled, depending on the killcount of each side, the entire Red or Blue team would WIN the war of multiplayer?
What if we had a complete side story with cut scenes, dynamic events and all that? Think Assassin’s Creed Revelations online. You would unlock cut scenes and story the higher level you got. I’m not saying to copy them, but definitely learn from that cool idea.
Or, even more crazy, what if what happened in multiplayer affected CAMPAIGN and the actual Halo lore? What if a BTB team who won a global tournament in Halo 4 would be featured in Halo 5, and their Spartan IVs fight alongside Chief in a huge battle?
Imagine, multiplayer with it’s own immersive story that is constantly being developed according to what happens in multiplayer, and the biggest things to happen in there would somehow show up in campaign.
TL:DR: Multiplayer has it’s own entire full blown story, where every player action causes some sort of repercussion, and the biggest multiplayer happenings would minorly affect campaigns.