1. Return of Dual-Wielding.
Dual-Wielding was introduced as a effective tactic in both campaign and multiplayer. In Halo 3 Dual-Wieldable weapons had less of a role and were optional in both campaign and multiplayer. The focus was on single wield weapons though Dual-Wieldable weapon were around if you pefer them. It was disappointing that Reach didn't keep the Halo 3 mentality of Halo 3's Dual-Wieldable weapons are there if you want to use them.
2. All Weapons in Multiplayer.
Every weapon from every Halo game for multiplayer or at least have the option to add though weapons in forge. Who cares if it doesn't fit in with Halo 4 campaign universe? Same goes with the maps. Who cares if it doesn't fit in the Halo 4 universe? If it's fun to play I certainly don't.
3. Human and Covenant Weapons (Possible Vehicles) from Halo 3 in Halo 4 Campaign.
In Halo 3 there was a cut scene of Elites and Humans trading weapons and possibly vehicles since they were also present aboard the Frigate. The very same Frigate that Master Chief is still on after it was cut in half by the collapse of the portal.
4. All Permutations from Halo 3 and Halo Reach.
I'm sure I'm not the only one that would have preferred to customize their Halo Reach Spartan with Halo 3 Permutations so it would have appeared similar. I was vastly disappointed when I discovered there was no Rouge helmet in Reach. They bothered with helmets like Recon and EOD. Speaking of Recon the only armor perms from Halo 3 Recon armor is the helmet, no chest or shoulders. I'd like all Halo 3 and Reach perms as well as an expanded selection instead of armor to choose from.
5. Less Vehicle destructibility.
If you hadn't notices. With each sequel it becomes more hazardous to drive/pilot vehicles and there are increasingly more tactics added through weapons and tweaks of weapons making it more difficult to use vehicles and can be a deterrent to some players. In Halo Reach it was more effective just fighting battles on foot cause the vehicles had cumulative vehicle damage which is crap since it is more dangerous then ever to be in a vehicle.
Halo 1. Indestructible. Halo 2. Destructible though was linked to your Spartan's life. As long as you live that Vehicle can't be destroyed. There were also additional means to target Vehicles such as lock-on rockets. Halo 3. The same as Halo 2 destructibility though there were far more ways in bringing Vehicles down. Lock-on Pod Missile Launcher, Plasma Pistol and Energy Drain kills engine, Spartan Laser, Mines. Halo Reach. Horrible Vehicle damage. The vehicle damage is no longer linked to the Spartan driving/piloting it and has accumulative damage and will blow up with enough damage even when you have shields/health. Plus more ways to destroy it. Plasma Pistol and EMP Grenade kills engine. Return of the Spartan Laser that wasn't in Halo 1 & 2. Plasma Launcher. Sniper Rifle does alarming amounts of damage to the vehicle. Sniper Rifle does far more damage to the Vehicles then any of the previous games. Why? Doesn't make since that the Sniper would get progressively weaker from Reach to Halo 3.
Halo 1. Baseline, bullet passes through targets. Halo 2. Doesn't pass through targets anymore effectively making it more dangerous. Halo 3. Passes though target again though bullet travel distance was introduced. Halo Reach... that is based before any of the other games. Instantly hits targets like in Halo 2 while having the penetrating ability of Halo 1 & 3 and damages Vehicles like an AT Rifle. Sure it could be that kind of weapon though it's contradictory to argue that since the Sniper Rifle is practically the same design and spec through every game. Conclusion, return Vehicle destructibility to the driver's/pilot's life instead of this crap continuous damage like in Halo Reach.
6. Revamed Maps.
IMO Halo 2 had the best maps of the lot. I'm tired of all these new maps they make with each game and only a couple from previous games. At least we can attempt to rebuild them in forge.
7. Every Vehicle from Halo 1 2 3 Reach in Multiplayer.
If you're like me you missed the Chopper from Halo 3 when you played Reach.
8. Some aspect of continuity.
Example.
For the more logical players out there you would have noticed Master Chiefs armor changed from Halo 2 to Halo 3 with no explanation. There was no opportunity to upgrade or change his armor as he jumped down to earth in the opening cinematic of Halo 3. Now in the Halo 4 trailer his armor has changed again with no explanation once again and all of a sudden Master Chief has an in built jet pack that assisted his movement though the zero G environment. Sure it looks cool though there's no explanation.
Time for a rant, "What? This is the same armor at the start of Halo 2. Why didn't you use that at any time in Halo 2? Would have been helpful when you took that bomb to that Covenant ship in space. Why didn't you use that to slow your fall back to Earth at the beginning of Halo 3? You could have used in anytime during the Halo 3 campaign MC. There were a few handy opportunities... and don't give me any crap like you didn't notice you had an inbuilt jet pack till the second Cortana woke you up from crio in the Halo 4 trailer. She would have noticed it when you acquired her from high charity." End of rant
9. Elites in Multiplayer.
Who cares if Elites could possibly not be in the campaign..? and give them more customization like they had in Halo 3. None of this preset crap in Reach. Plus all Armor from Halo 3 and Reach.
10. Machinama assist.
Save Films and Theater are great tools. Though some animations you try to get like the seemingly random death animations are sometime a time wasting experience. I'd like tools like camera shake and the like. Probably won't happen so there's no real point in continuing.
11. Unarmed Melee attacks (Not assassination animation).
Every since playing Dead or Alive 4 Spartan I'd though unarmed combat would be a great addition, at least as an optional choice. Assassination animation from Halo Reach doesn't count.