I have a few things I'd love to see for Halo 5.
1. Re-center the weapon aiming reticle back to the middle of the screen. Just like it was in Halo 1. This has been a pet peeve of mine since Halo 2.
2. Bring firefight back and combine it with player made forge maps. Think of the castle defense scenarios!
3. Increase ammo carrying capacity to Halo 1 levels. It makes no sense to only have three spare mags for a weapon.