i'll admit that these are selfish and based around stuff that pisses me off about the changes and additions to halo over the years.
Armor lock should not protect you from stuck grenades. i've lost tons of kills because someone i stuck with a grenade went into armor lock. if you're stuck, armor lock should only contain the blast to yourself and protect your allies. you should still die. also, you should stay in first person view. no 360 degree panorama for you to plot what you'll do when you exit armor lock. just stare at the fucking ground.
sprint needs to last longer and take longer to recharge.
jetpacks could last longer, but be less precise and take longer to recharge.
hammers need to have their lunge ability either severely shortened or, preferably, removed altogether. the point of swords vs hammers was that the sword lunges, tracks, and is an instant kill with an extremely short area of effect while the hammer has a very large area of affect (splash damage) but lacks the lunge and tracking.
the booting system needs to be changed to account for genuine accidents. there is no prefect system, but a good way of doing it would be you get one warning for your first betrayal, and on your second it's up to the person betrayed. all of them should still count against your score, not your kill/death ratio, though.
remove reticule bloom for everything but precision weapons.
add odst's free roam.
matchmaking firefighting. not like it reach, but like how odst should have been. i hate the 15 minute time limit. i want to enter a lobby with 3 people i don't know and play steadily stronger foes until all of us run out of lives and die. (i haven't played reach in a few months. maybe this has been added. i don't know.)
using campaign skulls isn't as fun when you have them to begin with. i wanna look for them. also, extra settings to the blind skull would be cool. it would be interesting to play with a limited hud.
more new weapons.
if there is gonna be a bruteshot, let it be the halo 2 bruteshot.
bring back duel wielding needlers, but with a shorter range, slightly smaller blast radius, and maybe a smaller ammo reserve.
i absolutely love reach's melee system (provided it's the same it was the last time i played). if you have even the smallest amount of shield, melee damage should never bleed over into your health. if this is still the case, keep it. if it isn't, bring it back. NOW!
make assault rifles deal more damage, but have a much shorter range. assault rifles are always less useful and outclassed by Battle/Designated Marksman Rifles. this needs to be fixed.