It seems you're taking suggestions...
Here are some critical ones...
> Keep each class at their base health. No ridiculous thousand-HP buffs
> Keep Zombies at melee only. That's kind of the norm for Zombie gamemodes.
> All Sentries should be Mini-Sentries only (this is probably already implemented)
> Ammo should be normal.
> As a Human, the Atomizer allows for infinite jumps. This results in him being untouchable by Zombies in open areas, making him very overpowered.
> Allow Scout to double jump with his other weapons (is there a reason why you don't?)
> When there's only one Human left, give the last Human a large health buff, as well as mini-crits on all weapons
> This is likely more of a mapmaker's issue, but areas where we are kept in one spot for a very long time before the door opens should be wide open arenas (Jungle Lab is the best in this regard). Mobility's very important in this gamemode, especially when there's low player counts.
Now here are some ideas for weapons...
> After hearing the Frying Pan's noise, I thought, maybe it could be used as a distress signal. Zombies hit by a Pan would emit a very wide and loud beacon effect for several seconds that would nullify sneak attacks
> As a Human, the Sandman's balls could full-on stun Zombies, keeping them in one spot for several seconds. In contrast, the Zombies' Sandman balls could have the normal slowdown effect
> As a Human, The Sun-on-a-Stick could ignite Zombies, on top of its other effects
> The Soldier's Grenade taunt wouldn't kill him instantly, but instead leave him at one health, and strip him of his primary and secondary for several seconds. That way, people who use it unintentionally don't cost their team the round.
> The Axtinguisher could one-hit kill burning Zombies
> The Powerjack's speed boost would be greater, but its firing speed would be slower
> The Stock Stickybomb Launcher can place 10 stickies at a time
> As a Human, all shields have increased turning ability
> As a Human, the Half-Zatoichi could heal some health with each hit, and it would stack with Mad Milk
> Lunchbox items would hurt Zombies, specifically the amount of damage they would normally heal
> Fists of Steel would resist melee damage by a lot, but movement would be slowed, and weapon switching animations would be slowed down as well
> The Gloves of Running Urgently and Eviction Notice would have a greater speed boost, and have mild resistance to melee damage to counter the health draining
> As a Human, all wrenches (except for Gunslinger) would deal additional damage and mark Zombies for death, on top of their other effects
> All Medi Guns' healing is slowed down, and Ubercharge (except for Kritzkrieg) is disabled
> As a Human, the Vita-Saw drains health from Zombies, and marks them for death. As a Zombie, instead of draining health and marking the target for death with each hit, killing a Human with the Vita-Saw fully restores a Zombie's health
> The Huntsman has twice as much ammo, and arrows cause Zombies to bleed
> Jarate has a slight speed debuff
> The Bushwacka could one-hit kill Jarate'd Zombies
> The Shahanshah grants you a speed boost (on top of its other effects, of course) when health is below 50%
> The Ambassador's accuracy is fixed, and headshots one-shot Zombies
> As a Human, the Conniver's Kunai does not reduce max health to 70, but rather, keeps max health at 125, but it marks you for death. A successful backstab still brings health up to 210, though
> As a Human, Your Eternal Reward still silently backstabs enemies, but instead of disguising as the victim,
> Zombies are allowed to have Sappers
Those are some ideas, feel free to implement them to the best of your ability.
Thanks for listening! Good luck moving forward, and I look forward to this gamemode's success!