A few ideas:
Character select
More settings for perk limit, perma perks, difficulty, buildables, power ups, mystery box, and some map settings to name a few, all could have some serious potential and can make the game infinitely more fun depending on how the host wants it
A more BO1 styled map list (although I miss the globe)
If it's ever possible, adding all the maps and weapons from WaW and BO1 onto their respective maps mixed with BO2's
Built in achievements and other form of rewards because why not
Unique factions for the survival maps
This ties in with the map settings I mentioned, but some more options for the special rounds on all maps would be neat. For example maybe TranZit could have hellhounds or Buried's witches could start being more threatening and come out from the house on their own. A few fun ideas that are entirely up to the host.
Death machine powerup restored
Maybe bon fire sale on all maps, make it like super rare, since it would be very cool to see it on another map other than five
If perma perks were added back via a toggleable option, maybe you could reimagine those too. Just some ambitious ideas.
There's definitely more but these were my top 10 that I could think of. I understand some of these might just be way too ambitious and therefore impossible, but I'd love to be surprised