The first thing I'd like is better armor abilities. Promethean/Forerunner Vision is a pretty good concept now that they've clarified it, but it seems like it wouldn't be useful to experienced players. Jetpack is more like an uphill battle. If someone stands under you blasting away with a BR, you're done. The concept of Hologram is great, but I seldom see it used effectively. Most people just send the hologram out in the middle of a huge clearing in attempt to throw off the enemy. Armor Lock was almost useless unless you activated right as a ghost or warthog was about to splatter you. Evade shouldn't be an armor ability. I'd hate to see it in Halo 4, but if they do they should make it similar to how they made Sprint.
Next, I'd like them to bring back the armor unlock system from Halo 3. I felt accomplished when I finally unlocked the Security helmet after grinding for gamerscore. Collecting skulls was tough, but rewarding. It doesn't feel so rewarding when I know I'll have enough credit to buy it if I play x amount of games. To keep other people happy, maybe they should implement both somehow.
Also, It'd be great if some of the Spartan Ops missions were Firefight-type missions, seeing how they chose to swap it out. (I'm unclear on this: will Spartan Ops "episodes" be replayable after new ones come out?)
Last, I'd like achievements for doing quirky things in multiplayer to sit this game out. There's nothing like losing a game because a guy on your team keeps getting killed trying to splatter someone in a Mongoose that gets my blood boiling.
On a side note, for preorderers/Legendary holders, there should be a code that gives you an exclusive weapon camo or something new. As cool as flaming helmets are, It'd be nice to see something different this time.