My suggestions:
Fix the rifle positioning. The Scar in your screens looks way off
Make several types of zombies. Fast and weak, super strong but slow (headshot only), and then work on dismemberment.
Make sure to have deployable Sandbags and ways to refill ammo
Try to have a good variety of melee weapons
Please (for God's sakes) have landmines and remote detonators
(Big suggestions)
Seek and Destroy gamemode - Where you and your fireteam/strike force have to eliminate zombies in an area strategically
Realistic grenades. No more stupid area damage script, actually firing off tons of shards *and* area damage. Also, grenades need to roll and bounce...
"Noob" weapons (Miniguns, turrets, cannons, and launchers)
Different armors. The ones that are currently available all look a bit too clean for a zombie invasion..
Oh! New CTF mode. Capture the brain.
Not necessarily, but make the Zombies try to steal the "brain" in the middle of a held down fort, and the objective is to stop them from stealing it.
Containment mode: Not Invasion, but where you have to keep the zombies in a controlled area for a certain amount of time (then at the end, say, 5 minutes, decide who wins or loses and start over)
Just ideas. It's all possible, but can be very difficult