Skill set. If you start at level 1 online you are level 1. Whatever you have unlocked stays the same. This way you control what level people you play with, lower levels can only play with lower levels etc... This would keep higher levels from griefing low levels and low levels from playing with higher people so they are carried through a match.
Or do away with having a level in single player and only have an online level. If I remember correctly Titanfall, SW Battlefront, and Battlefield did this when they introduced stories or single player into their games.
No idea what this means "play co-op online sounds like a pita".
They should have two different leveling systems. One for Offline/Solo and one for online Co-op. I am at level 40 even though I have not played one game of Co-op. I played each map on each difficulty. I do not play online as I hate pugs. I do not want to join a game just to hear 11 year olds (this is for 1-99 aged persons) cussing and screaming at you if you screw up or are not playing the way they want.
Another way to fix the online is to allow clan/guilds and/or private servers. This would also alleviate hacking/cheating. Since all files are client based and not server (correct me if I am wrong). This could also delve into clan competitions. Who can clear the maps faster using only a spoon etc...
More maps of course.
Military and Police grade weapons, M249, M60, Maxim, Lewis gun, more sub machine guns, stun, gas, concussion grenades. along with some more common firearms or even old school bolt actions (which would suck in a swarm)
Allow for you to choose your weapon set up before match
D.C. Grand Canyon, London, Paris, Istanbul, Cairo (with Zed Mummies) Alamo type ( where you fight wave after wave defense, and the map gets smaller and smaller) Aircraft Carrier or transport ship, swamps. Some large maps. Maybe one where you are driving a vehicle and run over zombies.
Please no skins. I do not understand why skins are so important over game play or missions.
Weapon Exp. the better you are the faster reload, hip shooting, quick aim, special ammo.
New characters and/or the ability to stay one character, maybe allow us to create our own character to play once we complete the episode(s) Nothing major just hair, eye color, clothing, race, gender, or play as one character through all of the Episodes, if they die or mission fail you have to rescue them to get them back as playable.
Ability to call in air strikes or arty (large map only)
More freedom and the ability to go stealth (also a possible mission) maybe you got separated from the group and have to make your way back to them.
Decoys to make swarms go toward a kill zone you set up with your defenses. More defense options based on class system.
Unrestricted defense placement. It sucks when I can only set up an auto defense one way and it is pointed in a direction or behind a building that makes no sense.
Traps, spike wall, pungie sticks, turnover gas barrels for fire traps, a rope trip line where the whole first part of the wave falls over ( that would be funny)
I have to say I love this game, and is far better then most Zombie games I have played.