I'm sure some [most?] of these have been mentioned elsewhere, but for me:-
Sandbox 'Home' Zone with Portals to Maps, MP Challenges and 'Housekeeping' areas
Housekeeping: Class Trainers, re-Appearance, Theatre incl. Observer mode, Leaderboards
Class Trainers: Nothing too OP, but choice for Starting Pistol, Grenade / Equipment type and one Perma Perk effect?
Custom Game Options: Choice of WWI, WWII, Cold War or Modern Weapons, choice Wonder Weapon, choice of Tacticals, choice of bonus round (none, Hellhounds, Monkeys, Minions etc), Difficulty (Easy, Original and Hardcore), Game Type: Survival, Grief, Turned, Gun Game, Kill Game (free bonus each round to player with most kills) and choice of Boss Zombie (Warden, Panzer plus new)
Personalization Packs for Zombies
3 mini-DLC with re-mastered versions of the W@W, BO and BO2 maps
Two map types - expansive EE / campaign maps and claustrophobic dungeon / building 'crawl' maps... comes with one of each and one each with DLC
New Wonder Weapon and 'wonder' Tactical... I like the Staffs, but something more 'weapon' than 'wand' would be better... maybe an option to build four gun options (bullet, energy, explosive and sonic) with four attachment types (grenade, shotgun, target-finder or reserve clip maybe?
Lots of ideas, but am beginning to ramble... may expand on these more later.