Multiplayer:
It would be awesome to play as elites again, and possibly playing as promethians.
All Gametypes with Elites.
Headhunter gametype.
hog potato gametype.(where you have to drive the warthog for points and more people in the warthog gives you more points)
Armor lock.
A gametype where if you kill someone it gives your team +1 and if you die its -1 but has limited time
UNSC Flying Vehicle.
a mantis but in a sort of covenant style.
Forunner vehicles.
Campaign:
Arbiter.
Cortana.
Chief.
Large amounts of marines and odst's when in need of assistance.
a team of spartans.
Forge:
Magnets.
Precision forging.
Delete all of these feature.
Delete by pallet feature.
make a night time Special FX for daytime maps and daytime Special FX for nighttime maps.
different colored blinking lights, not just red and yellow but other colors too.
higher budget or lower amount of money for each forge piece.
higher amount of usable forge pieces.
locking feature.
Spartan Ops:
Better Storyline.
Different personal loadouts than the multiplayer personal loadouts.
more episodes/seasons.
More map variety.
FireFight:
More action.
more enemies in one round or more enemies on the map at once.
custom loadouts.
This is all i could think of at about 1:20 AM because i was tired.
I'll add to it if i think of something else.