Personally, I think Aetherium upgrades, rampage inducer, applying blueprint attachments to wall buy weapons from CW should return. and maybe just maybe a modifier to a perk of your choice like Tonic from BO4. Points earned from shooting zombies should be better than what we currently have in MWZ. Speaking of MWZ, choosing what lethal and tactical to start with should return with some restrictions to which equipment you can start with. The most important one I want to see is the pre-BO4 quick revive where it revives you instead of faster health regen (faster health regen should be included in quick revive too).
But, these are just my ideas. I would like to here from yours and if you agree with some things i said
The only thing I care about is the aesthetic. I mostly stopped playing CoD for the multiplayer after AW, but what kept drawing me in to the series was the third mode having alien theme parks, haunted mansions, frozen magical ruins... If BO6 is just dudes spouting "EKIA" while mantling around a grey building, what's the point?