Going to get straight to the point here. These types of things will fix the immersion of Halo's multiplayer, and probably for the whole game in general.
- Naming classic weapons their original names, such as "M6D Magnum" instead of "Halo CE Magnum", this can simply be fixed by naming weapons by their correct name. Naming them by which video game they were in just sounds like the game is backtracking too much and you lose a sense of being in the game you're playing, such as H5.
- Map design. Forged maps by the community are one thing, but it shouldn't be the basis for every BTB map. Nor is making everything look like a war simulation, such as in warzone. Arena maps just looked like they were made for spartan simulations, not real-life places. I don't want the maps to make it feel like I'm completely in a simulation, I want to feel a part of another world or map all-together.
- Art style. I know this is already being addressed, but it also helps with map design. A lot of the objects in H5's forge were plain, and a lot of layout pieces for built-in maps were overused in Arena. Adding grit and character to objects and places gives it more depth. Everything looked too plastic in H5. The same goes with forerunner tech, not everything has to look super clean and over-the-top. Halo 1-3 did this very well in my opinion.
- Getting rid of some spartan abilities (at least). Notice I said "some". It doesn't have to be everything, but spartan charge and ground pound need to go. They seem to be a nuisance in the gameplay of H5 and also seem a bit unrealistic (ground pound more than spartan charge).
- Adding blood, mud, and grit (Modod)
- Getting rid of hit markers (Hot Chili Fries)
- Getting rid of cheesy spartan callout dialogue -- making it optional (M killshot)
- Getting rid of weapon spawn callouts -- maybe for some game modes (can i have host)
- Getting rid of goofy/childish cosmetics, such as some armor effects and/or some weapon skins -- Example: Pizza skin (TriedCrowd418)
- Getting rid of sniper glare -- unrealistic for 2500s and lowers skill gap, can be replaced with bullet trail or smoke (King of Brunch)