I know that theres only 3 game modes but i think that they need new ones:
CTF: Capture the flag. Capture the enemy flag to score for your team.
INF: Infection. On start the first zombie will be the master that haves one millon and normal zombies would have 100 HP.
DOM: Domination. Dominate the controls A, B and C, If you see an control, Go dominate it.
GUN GAME: Gun game. Kill an enemy to upgrade your weapon, you cant pick up other weapons becaus you will get kick.
THATS ALL: I dont know what game modes that i can add.