This is what I would like:
MULTIPLAYER
1. Playable Elites, Brutes, and Promethean soldiers
2. Classic gameplay.
3. No REQs or at least make them like the credit system from Halo Reach.
4. LAN support
5. FIREFIGHT!!!!
6. Bring back Warzone and add some new gamemodes like Warzone CTF or Warzone Infection where you fight off waves of Flood and whoever dies gets to respawn in the next round.
7. Large maps like Valhalla and Blood Gulch.
8. Remakes of old maps like Guardian, High Ground, Last Resort, ETC that are dev made.
9. Vehicles in Slayer
10. Less armor that looks very different and recognizable. Also have armor be unlocked by completing challenges like earning all the achievements in the game or beating the campaign on legendary.
11. Halo Reach style customization and the ability to customize your HUD.
12. Darker maps like in the Halo 5 Beta.
13. Bots
CAMPAGIN
1. Old art style. Since 343 likes to give "Lore Reasons", they could say that Cortana EMP'd everything which makes Spartan IV armor not work anymore so they would have to use old armor from Halo 3. Also, if 343 brings back classic Halo then they could say that the older armor is heavier than the new armor so it would make the Spartans slower and make it harder to sprint.
2. Character deaths. If there were character deaths are in Halo 6, it would make the game more emotional. Also, if 343 had Cortana kill Linda or Kelly it would make Master Chief want to avenge her by stopping Cortana. It would make him think "She needs to be stopped!"
3. Fighting on The Ark and Installation 04C.
4. Return of the classic soundtracks. I would also like to see "117" and "This Armor" return.
5. Better Ai. Enemies should be running around, dodging bullets, running into cover and dodging grenades. They shouldn't be just standing there like bullet sponges like in Halo 5. I would also like to see when an enemy gets shot, they should be knocked back or maybe have a hole in their armor.
6. More blood. Have lots of blood effects like in Halo 3. Something I think would be cool is if you melee and enemy up close, blood would splatter on to your VISOR.
7. Add more Promethean vehicles. Instead of having them just teleport into the level, they can have their own dropship which disassembles itself and reassembles itself into Promethean soldiers. Also, if you shoot of some parts it makes less Promethean soldiers to fight. I would also like to see a Promethean equivalent to the warthog and scorpion.
8. Simple story. I shouldn't have to read a book or watch a TV show to know who characters are. Just give them short and simple backstories in the game and then write books going more in depth on who the character is.
9. More destruction. Nothing satisfies players more than watching phantoms exploding into pieces or scarabs exploding. Also the phantoms should explode similar to the way they do in Halo 3 where you can walk inside the wreckage and all look the damage you've done.
10. More vehicle assaults and large open levels like Halo from Halo CE and The Ark from Halo 3.
11. Equipment from Halo 3.
12. Marines that look militarized. Marines with their green armor and their rank on their arms would make them look like actual soldiers.
13. Space battles.
FORGE
1. AI in Forge
2. Forgeable Firefight maps.
3. Pelicans and phantoms in Forge.
I know that 343 can't add everything in this list into Halo 6 but hopefully they take some of it into consideration.