(LONG READ AND CRAP SPELLING)… you have been warned 
Sorry for bad spelling+Grammer-Dyslexic-Sew me!
Not all of us in the halo community are savage beasts of the MLG (jokes) and we like to have fun in Infecton, map building, being a rebel and gliching, killing our best mates in a not so friendly game of slayer or invasion, Its called Custom games! here are some of my ideas for halo 4 to make it ever so great! 
Contense:
.Fire fight
.Forge
.Weapons
.Vehicles
.Custom settings
Fire fight
I suggest that in the options for custom fire fright, where you can select what waves will arive (like in reach) that there are 2 teams:
UNSC
COVI
And during this you can select what teams will take part in the battle and what units will appear on each wave, As shown below (also take note in being able to chose vehicles as a unit instead of a squad);
[UNSC] -red Team
1st wave:
- marines
- Sniper Team
- warthog unit
2nd wave:
3rd wave:
Bonus wave?
[Covinmant]- Blue Team
1st wave:
- Grunts
- Jackels
- Elite with Ghost
2nd wave:
3rd wave:
Bonus wave?
This way you can play fire fight in the view of an elite and if possible make it a 8side team game like Invasion?
Also a good option to have would be how the units arive, Explaind below;
Wave 1 options
1.Grunts -> Phantom drop ship
2.Jackels -> Spirit Drop ship
3. Elite with Ghost-> Phantom drop ship with Banshee escort.
1.Marines -> ODST drop pods
2.Sniper Team-> Falcon
3.Warthog Unit -> Pelican with Hornet escort
I think the ability to chose from two teams in fire fight would crate heuge battles- you wouldnt even have to take part… you could sit in the corner of a map and just enjoy the view of a heuge scale war!
Forge
I under stand the whole argument about how forge worlds and forge maps take up a lot of memory and console enegery (or whatever) so how about an optional forge world to install to your xbox upon launch of halo 4? This map would be heuge and not many restrictions due to memory being saved on your hardrive!
this map would take place on earth neer New-Mombasa (or however you spell it) in the dessert- much like in levels 4 and 5 of Halo 3… Travico high way… or somthing like that 
In this forge world you can get every vehicle, turret, crate, barrel, weapon, wall, fence barricades you can see from any halo game belonging to the UNSC or Covi! so no more grey walls! the walls are made up from snippets of maps like power house, and the mission on halo 3 just before the flood arrives (sorry name slips my mind)
Weapons
I think the ability to place down a large verity of weapons in forge for you custom games would be amazing, it would help make many good game modes and allow difforent experiences in custom games and custom games only!
I think to save time and avoid the fact 343 cant be botherd or dont have enough time to make new weapons… they could coppy a weapon such as the assault rifle and give it a 60 round magasine and a scilencer to make a spec ops rifle with basicly no effort but a small skin change, make it not appear on raidar when shooting and a larger magasine size!
outher ideas are:
.vairients of the smg, Normal and ODST smg.
.Dmr and Battle rifle.
.Brute plasma rifle (a red one for those who dont play halo2) and normal.
.a batton or night stick (works like a sward) so you can make cool cops and robbers game modes!
.the shot gun but with drum mag, slightly faster fireing and lowerd damage.
Could go on all day!
may i say new weapons dont need to be balanced or fair! they could be just for custom games!
Vehicles
Post your ideas below and see the list grow!
the ability to change collour like with the mongoose!
Just from every halo!
warthogs-guss-troop-scout-rocket-pc halo rocket hog ect…
Different shade turrets-Fuel rod- AA- halo ce shade turrets too!
Banshee-herotic banshee with no banshe bomb but faster firing plasma!
PELICAN+ Phantom
CIVI VEHICLES!-police warthog (no turret and flashing lights) - Gangsta warthog(cool paint job, go faster stripe and spinning rims) - forklift- work trucks and lorrys-
Ghost woith duel needlers and green paint job
Destroyed Vehicles and crashed pelican/phantom.
Post your ideas below and see the list grow!
Custom Game Settings
Yea its not a verry long list, but it needs to be said!.. 
The ability to make a player not drop a weapon when killed… so pesky humans in infection dont steal swards >:(
Post ideas below and see the list grow!
Sorry for bad spelling+Grammer-Dyslexic-Sew me![/u