I've recently been thinking, last year, when 343 industries announced 'The Reclaimer Trilogy', we jumped to a sizable conclusion that this means there will be a Halo 5, and Halo 6, presumably separated by a year or two. With Spartan Ops being as extensible as 343i are saying, I was thinking that it could be possible to extend the campaign, let's say every year, maybe redefine some assets, add new weapons/vehicles, etc. So rather than having Halo 4, 5, 6. All of the campaign could be playable from Halo 4, where the alternative campaigns can be purchases as either XBL DLC or Disc content, like the Halo 2 map pack.
This way multiplayer could also be given 'title' updates, adding new maps (as regular DLC), and various changes that come from game to game.
Just a thought, I thought it was an interesting concept. I'm not saying I think I'm right, though.