All they need to do to make halo 3 great is:
1) Increase back to 90 degrees Field of Vision. I Fing hate halo 2's tunnel vision.
2) No rocket lock-on on land vehicles; just lock-on for banshee.
3) Battle rifle with semi-auto single round burst; not 3 round bursts (more like halo 1's pistol). Need a human weapon like this, instead of the covey carbon.
4) Fall damage.
5) Smoother animation like halo 1.
6) More natural feel, like halo 1.
6) No lock-on/lunge melee.
7) Huge environments like halo 1's; where it makes you feel small in enormous infrastructures.

Leave bullet casings on the ground permanently, or longer like halo 1.
9) Thats all i can think of for now.....
Hell, i would be happier then poo if bungie just remade halo 1 with better graphics, a few bug fixes, and online support (too much bullet lag with the current tunneling software out).