Honestly, I look forward to seeing whatever 343 wants to throw at us, and judging from how well they've done simply by adding content to Waypoint, I expect great things. I do, however, have two personal suggestions of what I'd like to see added for Halo 4.
1) More customization for Multiplayer-Character:
Obviously, the Master Chief is pretty iconic already, so I can understand not wanting players to tinker around with his appearance for the Single-player Campaign. But when it comes to Multiplayer, I say the more customization, the better -- without getting too silly, of course. Myself, for example; even though I know it wouldn't be added, my "dream character" would basically just be kind of a self-portrait, a "Spartan-ified" version of the USMC Dress Blues (we may not be the brightest, but we dress the sharpest!) with red hair. A bit narcissistic, I guess, but the point is this; I like having a character I feel like I *am*, rather just "just an avatar". Even without foregoing the helmet, I'd love to be able to strip off my Spartan's upper-sleeves, and simply have a few options for his skin-color.
As well, I feel like the Elites really got shafted in Reach. I'll admit, I like the thought of them using one-piece sets of Armor in contrast to the Spartan, given their culture is more about tradition than efficiency, but I'd definitely like to see more added, as well as more diverse means of acquiring them. While Spartans might buy their equipment with Credits (in-game currency), perhaps Elites would only acquire sets after completing multiple achievements, which works as well for gameplay as it does Story. And please, for the love of all that is Good in this world, let Elites play in standard Matchmaking, not be restricted to special gametypes only!
2) Custom Campaigns:
Forge already laid the groundwork for some pretty intense customization... but sadly, it was restricted purely the Multiplayer maps. But what if, in addition to what Forge already has, you could place Objective-markers, Enemies, and Ambush/Patrol markers? Especially within the actual Campaign levels? Granted, nobody is going to be making levels of the same caliber as a Triple-A studio, but some fun stuff could still be done. Imagine doing a Stealth speedrun, with only a Pistol with a single clip to last the level? Or racing against other players, armed only with Rockets and Invulnerability, relying on Rocket-jumps to speed you along?
I'm sure there are people who could come up with even more creative examples than that. It might not be the same level of tools as LittleBigPlanet might offer, but inFamous 2 even saw fit to adding a similar feature, and although most of the customs are garbage, there are a few genuinely fun missions. Maybe you could even do a "Custom Campaign of the Week", giving a one-off Credit bonus for completing the level, and giving the Author an exclusive piece of Armor for his/her Spartan or Elite?
Again, those are just *MY* suggestions. Any thoughts or comments? Feel free to add your own, expand on mine, or even just talk about how delicious twinkies are.