Just two key things that would have me shovelling cash out of my pocket like a fiend...
1. Higher player counts. Oh how I TIRE of 8v8 matches. How many iterations of Halo do I have to put up with before we can be treated to something reminiscent of an actual war?
Halo works best in BTB scenarios, when every player on the map is converging on one open vista, with vehicles whizzing around, bullets visibly flying across the map and s#!% blowing up everywhere. Only rarely do things come together perfectly like this, but when they do it creates a feast for the eyes.
Imagine 32v32 player matches? or 64v64? or 100vs100? Crazy battles would be breaking out all over the map. It would be Halo multiplayer fully realized. And it would inject a much needed breath of fresh air into what is starting to become a tired and predictable formula.
Not to say that I'm not loving Halo 4's MP, but I think in the next generation of consoles we will be at that point where the technology will be capable of bigger servers. And if we don't get it I'm going to be royally peeved.
I mean, these are "war games", not "skirmish games".
2. Dynamic, destructible environments. This is less of a concern, considering that destructible environments isn't what Halo is about. But, if 343i were to take a page out of the Battlefield playbook, this would be the ticket.
The best thing about Battlefield is when your in the middle of a firefight, and bricks of cement are flying around, panes of glass are breaking, paper is exploding from mailboxes and generally environmental details are coming apart.
This always adds unbelievable amounts of immersion and awe. Halo doesn't necessarily need to have important structures crumbling, but some sort of middle ground will add a lot. The next generation of consoles will certainly have the capacity to deliver on this. With all that RAM and CPU power, particle details should be abundent. Something that is undoubtedly missing from the Halo universe.
Everything else is OK with me. Though, I'm hoping for unpredictable innovation, as well.