Things i want: A better type of ranking system (maybe 1-50 again?), better maps (Halo 1-3 all had a good amount of classic maps, Halo Reach/4 not so much), more maps, less Armor Abilities, good gun balancing between BR/DMR/Carbine, another AR type gun (no smg though, that gun sucked a lot), a good variety of multiplayer playlists, more guns (if it would benefit Halo), Spartan Ops, Firefight, Brutes again, dedicated servers, fixed melee system.
Things i dont want: Flood, boltshot or any type of close range gun besides the shotgun/Promethean shotgun, ranking system dictated by experience, Halo 2 lunge on the sword, useless guns, class loadouts,
I cant think of anymore atm.