General
Splitscreen
Make it so when playing 2 player splitscreen, you have the option to split the screen vertically rather than horizontally.
Hold to Aim
Add the option to disable to “hold to zoom, release to unzoom” mechanic. I am constantly unzooming when I am just trying to press it once.
War Games/Custom Games
Bring back Infection!
That means no armor abilities, one shot headshots with pistol, and last man standing can withstand 2 hits. Just like classic Infection from Reach.
Basic Slayer
Add a game type where you get to use your loadouts except there are no armor abilities, packages or ordnance. Just plain and simple gun battles with weapons you can choose.
Loadout Characteristics
Make it so in Custom Games, certain loadouts have different player traits. What if I want a swift class with high speed and low health with a pistol, and at the same time have a slow, high health loadout with a SAW? It would open up lots of new possibilities and would make Custom Games so much better and more amazing.
Playing as Elites
Come on 343, you know we all want the option to be able to play as Elites. It was possible in Reach in some circumstances and I believe in Halo 3 you can play as an Elite whenever you want, but no longer in 4. If you do add Elites, add armor to the Elites that you can use to show off unlike in Reach. Make it so you unlock the armor by completing Covenant specific challenges/commendations so that players can truly show their love for the Covenant.
Forge
Scripted Events
This would completely change Forge as we know it and make it something totally amazing. There would be SO MUCH more possibilities and potential for amazing content. Take a look at TimeSplitters: Future Perfect. It would be like that but more sophisticated and advanced. There would be many specific triggers with multiple parameters able to be set for a single trigger. Be able to set variables which triggers can modify while also allowing variables to be used as triggers.
Forge World
Please bring back Forge World. It was so huge! You could build a large base in the Coliseum, have long distances bases in the canyon, explore/expand the caves, or even use the entirety of the Forge World as a large adventure map. Please bring it back. You will gain the ever growing love of the Forging community. And in Halo 4 it will open up lots of new possibilities. Do that, or build your own.
Themed Pieces
Add different themed pieces to Forge. Imagine a 5x5 block that was Covenant themed? Make it purple with green detailing? Or have Forerunner themed blocks with a silver, shiny look.
Hornets
Please, 343, add a Hornet or Pelican! Pelican seems too extreme but at least add a Hornet. But Pelicans would be amazing also. Falcons were so amazing in Reach, being able to transport troops from A to B quickly was amazing and was a great support vehicle. The Hornet would be even more amazing because it would have a machine gun for the pilot (I believe). Not sure why they didn’t add it in the first place. Do it, the community would be so happy!
Transport Warthog
Any of you guys ever seen the Warthog in the Reach intro cutscene? It is basically just a Warthog without a turret. I assume it is mainly used for cargo transport, but in battle it would be useful for transport. 3 man or 4 man transport at one time. Either would be awesome.
Colored Mongooses
Remember colored Mongooses from Reach? They were cool because they could represent team Mongooses. Bring them back. Not a major feature but would still be nice. Better yet, make all vehicles color-able. Imagine a red Warthog or a yellow Ghost. Or even a green Wraith. 
Fine Tuning & Zoom
For those of you who don’t know, in Reach you could very precisely move objects if Edit Coordinates just wasn’t doing you justice. And you could also get better precision while doing so and when just carrying large objects where the camera is far away with zooming.
Larger Budget/Cost Reduction
I have no idea why the budget is so low in Forge. Forge is capable of so much more the the budget allows. Over 10x more at least. Modded gametypes let you have well over 100x the amount than the game currently allows. That is 1,000,000 dollars compared to the measly 10,000 the game allows. I think 25,000 would be an amazing change to the budget. Even if it were 15,000 instead of 10,000 it would be amazing. Do that or get rid of the ridiculous prices of objects like railings being 50 dollars when a wall coliseum is only 10. It would cost about 4000 dollars to make a structure as big as a 10 dollar coliseum wall with railings.
Select All
Add an option to grab all objects in your world or select a radius to grab. It would just be such a major convenience and a feature there is just no reason not to add.
Spartan Ops:
Lag
Get rid of the lag! Whenever I play Spartan Ops online I always have about a half a second delay on my button inputs. Makes it impossible to play Spartan Ops online
Help add ideas to this list and request sticky (not sure if possible on Halo Waypoint) so 343i can see this and maybe implement some of these ideas and other people can add to this list! Who knows how long after Halo 5 is released they will stop supporting this game and the people who will not pay 500 dollars for the new console plus 60 for the game itself are missing out! Halo 4 has the potential to be so much more!
Hivemind is the closest thing to Halo 3 infection i like it.
Unless you have a movie theater screen, split screen would suffer if it did as you suggested. It becomes too narrow.
They could cut off the top and bottom edges like they do the sides now. Make both screens square. So you would have a “fullscreen” splitscreen experience. I think that would be amazing. Like on Call of Duty: World at War.
BUMP. I added more things to the list.