i think it is virtualy imposible to create the perfect game that everyone likes every thing about even if its halo cod or even gears, but some ideas that i think most people would like to be put in halo 4 are...
In infection (if you add it in the game) you should be able to change the way your infected looks like. For example flood parts comming out of the armour and a few smaller pieces those bursting flood bubles on your body.
For armour have the little the atachable extras either usable or have it so you can place it in different spots on your armour (a none functional knife could be moved to the shoulder chest pelvise etc.)
Have different selectable styles of knifes for assassination cutscene. Maybe different skins or even choices between a machet, a small energy sword or maybe even two small energy swords (not for use in game just for the cutscene thing on on your armour) Maybe even small guns for the assassination cut scenes.
Please comment below about any other ideas that cant go wrong.