Long time Halo 2 multiplayer gamer here, I really hope Halo 5 is a xbox one defining game.
Large scale Maps/60-70 player count
For me this is a must. You can still have smaller maps with small player counts, but also give us larger maps for the Halo gamers that want that. I much prefer larger scale maps where the action flows more organicly than small corridor maps. The Xbox 360's limited power couldn't achieve high player counts, but here's no excuse now that the Xbox One can.
Halo 2 cross team chat
In Halo 2, you could talk to the other team once you were close enough. This added another element to the Halo 2 multiplayer.
Dynamic weather
Rainstorms and clouds forming would be really cool.
Wide range of Map Locations
Urban maps, destert, jungle. Playing at the edge of a volanco, alien worlds with shooting stars going by in the sunset. Giant planets in the sky.
Halo 2 styled Custom Games
Tremors, Tower of Power, Hide and Seek, Zombies, Troy, Juggernaut were so fun. I played custom games more than regular multiplayer on Halo 2. It would be great if 343 gave the attention of detail in the setup rules of private games that Halo 2 did. That's the reason it worked so well and was popular in Halo 2. This was a great change of pace to the regular multiplayer.
Custom Gear vehicals/weapons and Armour
I want this to be more fleshed out in greater detail.
Destructible envirments
A little destructibility is much more realistic than none.
Classic Halo 2 DLC map pack
Coagulation, Colossus, Headlong, Warlock, Zanzibar. So many great maps from Halo 2.
Classic Halo Gametype