Considering how Halo: Combat Evolved was built to be a shoot-or-be-shot game with vehicular combat and several cooler things, certain things used in previous games might become more fleshed out and change or slightly improve gameplay.
-Vehicles. Yeah, more are better, but we can't have them all.
-Weapons. Just like vehicles, but having every one from every game (not each version of course) is within the realm of possibility.
-Armor Abilities. Things get tricky here. Armor abilities totally changed gameplay. I'd understand if they leave us with sprint and evade if anything at all.
-Forge. Map editing like a boss. Should have region based "Forge World" maps (i.e., Forest, Desert, Arctic, [Space?]).
-Player Customization. I liked the Reach version best. It had all the good choices. Maybe allow players to make their own armor pieces within a limited frame (can't be too big/too small).
-Online Multiplayer. Again, Reach took the cake. Between Competitive matches and Firefight or Campaign, it seemed pretty epic.
-Ranking System. This should be split between Halo 3 and Halo: Reach. I think you could earn EXP/Skill from all games (i.e., Campaign, Matchmaking, ect.) and Credits for all the actions you did in a game (i.e., kills, assists, ect.). Credits go toward the Armory and EXP/Skill goes toward your rank.
-Assassinations. A few newer animations, that's about it. Other custom animations for weapons besides the Energy Sword.

Some other things I'd like to see in game. Just some stuff that sounds cool.
-Advanced Forge. Switches (spawn/despawn) that can be linked to up to four different objects. Object naming for easy reference. Gravity zones (i.e., 0%, 25%, 50%, ect.). Similar Sapien abilities.
-Orbital Drop. *Top Secret* (I'm actually trying to work on this in Halo: CE. If it works for me, it should work for them).
-Epic Vehicles. Pilot able Pelican and Phantom. Pick up vehicles underneath with the push of a button, drop troops onto the battlefield (essentially all the stuff we already do).
-UBER Vehicles. Drivable Scarab and (perhaps) Mythos. I'd imagine that the Mythos would have treads and look similar to the Elephant with an Onager (Reach Mass Driver) on the back.
-Destructible Environments. Bring a building or two down with a few tank shells. Drop a rocky overhang on a few unsuspecting enemies. Go crazy.
-New Armor Abilities. If they decide to put in Armor Abilities, they should include everything in this list on top of what they have now.
+Self Destruct. Three-seconds audible countdown timer that kills you and takes out anything within a 2.5 meter radius. Cancel at any time.
+Teleportation. Point and click (like hologram) to teleport yourself anywhere across the map within the safe boundries (without bringing objectives with you). Recharge takes 30 seconds.
+Shield Gauntlet. Forward energized protective shield that operates just like a Jackal's shield and you can still carry (not use) two weapons. You jelly CoD?
+Magnetism. Walk on any surface (even upside down) with energized boots of awesomeness. I can't tell you how many times I wanted this on Guardian.
-Larger Campaign Encounters. Something like Halo: Reach's encounters. They had like what? 80 guys on the field at once?! : |
-Console and Platform Linked. Xbox players can play with PC players and vise versa.
-Forgeable Firefight Maps. Use one of the Forge World maps and create your own Firefight maps. Place squad spawns zones, dropship spawn/lz locations, drop pod locations, ammo crates, ordinance, health packs, respawn zones, and anything else necessary for Firefight.

All this and more, my friends....