When I say "mechanics", I mean the Halo 3 hitscan BR, too. Maybe reduce the spread at a range (keep it in, but make it a little more predictable at medium-long range).
If there's any mechanic 343 needs to take away from Halo 3 or any other Halo is the simple fact that less is more.
Halo 5 can be fun to play sometimes but as soon as you start designing for this game, you realize that it fails one of the major fundamentals of Halo gameplay: predictability.
You can go anywhere from anywhere in Halo 5 at any point. This was of course a problem as well when Jetpack was in the game, but even then players could not move as fast as they can now. Again, that's great in a Campaign environment or in Warzone where the maps are fucking massive and you want to "feel like a Spartan" charging down a beach or whatever. And there's nothing inherently wrong with faster movement.
But every game that does fast movement well either has health with health pickups or really fast kill times. Halo has slow kill times and recharging shields, so smaller player counts fall apart at the seams.
Honestly, I would trade Sprint, Thrust, Clamber, Spartan Charge and Slide for a single Double Jump that we can use at any point to propel us forward, upwards or backwards. If it was momentum based, we could have very intelligent level design and predictable player movement again without losing the "feel good" movement of Halo 5. And of course, you'd be able to shoot while doing it unlike the abilities I just listed. Sprint and Thrust will only ever be used defensively so long as you cannot shoot while doing it.
Most importantly, Double Jump would not break the spawning engine and pace of the game. That alone justifies the removal of the afformentioned elements which completely offset the cadence of Halo combat - a nuance that 343 wasn't able to restore with the Pistol. The maps no longer direct players; players just do whatever they want.
I know it's pissing in the wind at this point which is why I don't consider Halo 5 a real Halo title. Even Halo 4 retained a lot of the core aspects of the game (at least when you modded Sprint out). There are just too many fundamental elements of what made Halo work that have been lost in the additions. The game was more than "even starts and floaty jumps".
Every well designed map in the history of Halo would play like ass in Halo 5. All this talk of a Halo 3 remaster and all I see are people clambering all over Guardian or thrusting all over Narrows or Sprinting all around The Pit. Sure throw in the original Halo 3 gameplay on top of that if you want, but then you'll end up with H2A, which was at its basest less nuanced than the original game.
