: I love this idea. Allies come in and out of the story like power weapons or
: vehicles. Maybe even keep the main campaign as an entirely single player
: experience (or two Chief co-op for the oldies that insist nothing change).
: Leave the four player co-op to a cooperative side story campaign like
: Spartan Ops or Splinter Cell: Conviction's co-op mode and let that exist
: as your co-op mode alongside Firefight.

: - MacGyver10

You know what game used these ideas really well? Resident Evil 5. Unintentionally so, even. Your AI buddy is not very bright, so you basically lug them around with a weaker weapon to provide cover support while you do the heavy lifting. It remains a solo experience for the most part (except for the fat that you have to take care of a second health bar). This changes when things get serious (such as difficult encounters or boss battles), and you give them a good weapon and the order to attack.

Suddenly, you have a trigger-happy aimbotting powerhouse at your side, giving you a ton of help (as well as a distraction for the bosses) and healing until you complete the encounter, at which point you can call them back and they switch to their weakest weapon.

It is a pretty great experience that puts control in the hands of the player, and when you play co-op, you get a competent player throughout, but the catch is that now you have to share/split resources and communicate (such as this clip of Sammy and I tackling a particularly difficult encounter with dwindling supplies):

It definitely made the game way more fun that it had any right to be.. I can only imagine what a similar thing could for Halo (especially with something in the same vein as the cooperative melee combos in the above clip).

Otherwise, I agree. the games should be single-player combat puzzle rooms, rather than the slow, inexorable push forward that having three respawning AI/human teammates provides.