Towards the bottom left of the map, there is some train track that is obstructed by a poorly built structure. I suggest that the structure is replaced with two trains and outright removed. The two trains would be at different parts of the track as to create a tunnel to horde zombies into which could be a risk/reward scenario to bait zombies.
Add some form of a unique item or weapon per map. Maybe a gun/melee that only spawns on one map or a jump pad on one map or a new type of mechanic for another map. I'm not saying it needs to be drastic like a maze or an elaborate story line on every map, just something to add more character and reasons to play other maps.
Possibly allowing the difficulty to be changed per map before starting the game. So maps like Trainyard get more spawns or harder enemies.
A new zombie type? It could be something that disbands other zombies like one big tanky zombie instead of five quick ones in a normal wave. And mini-boss zombies that can sometimes spawn that give extra units once defeated.
Price things up in the store, or disallow the ability to buy better guns until certain rounds or removing it all together.
I don't know if this would be possible but having its special item do something other then kill? Maybe it could; slow down waves of zombies or turn them around, or lowers amount of zombies per-wave, or it could amplify weapon damage for all teammates in a circle, or give some sort of armor to teammates in an area.
Find some way to nerf its combo power and buff its alps, maybe by cool down, and then for alps three laser strikes?
Applies to some classes; I've noticed that some classes get their abilities really quickly, I don't know if this is possible but you could up the amount of kills on combos or implement longer cooldowns for special items. But not by too much as to upset the balance of gameplay.
(~Either way for a game mode that just released I'm content with it. Even if none of these changes are possible, I'm still satisfied with what you guys have built and think it's a stable balance of gameplay. But there are some suggestions I have. Hopefully, I don't sound like a bumbling idiot.)