I will Think about it during my spare time. i will try to Help You! ^^ Your map is realy good, so thatYOu must keep it save and impove it!
Try this.
1 Spell
Type: Passive, Boss Ability
Name: Blood Bath
Description: Everytime when The Boss loses his HP (When he's damaged) His blood will heal nearly zombies. Causes any (For axmple 20%) damage taken by boss to be reflected and changed and heal all nearly zombies in 600 AOE(For example).
2 Spell
Type: Passive
Name:Zombie's Bite
Description: Every zombie's bite will make a builder sick and reduce his Armor by 1. (Buffs stacks with one another)
3 Spell
Type: Active, Boss Ability
Name: Hunger
When bosse's Hp is on low Hp amount he can eat nearly zombie to restore some hit points.
4 Spell
Type: passive
Name: Plague
Atacked player will infected. And he will lose 1% Hp per second. All alied builders which contacted with infected player will infected too. Duration ~20 seocnds. (They infected^^ They have 2 choses => they must to stay far from one another,
=> Or just stay in a crowd of infected builders and infect each other Again and again. (Aoe of infection ~300) Hmmm... so cruel ';..;' =)) )
Or this,
Just An Idea: When zombie kills (Or eats^^) a builder, he will recover all his hp (or 50% of Builder's full Hp amount), And after that Zombie will Grow up and become bigger and faster or somethin like that

(Or builders Death will heal all nearly zobies)
Give them an ability to eat corpses(So disgusting =)). Or make them faster when they can smell a human with low HP.
I've found for you some interesting models. Link for download
http://depositfiles.com/en/files/818ep4fk6
http://depositfiles.com/files/x53fp1g2z
Try to play this Map. it's not bad at it's terrain. Maiby You'll find some ideas.
Have you ever watch Resident Evil? you can find a lot of ideas in this film. The best movie about zombies Ever. You can use some soundtrack from it. Just convert it to Midi and reduce it filesize. Or use a small part Of it. But i'm not profi at working with MusicFiles.
I have a model of a nasty meat worm. if you need i will find it in my archeve.
P.S. Work! Work^^! You've put your Hand on this project so that Rise both of your hands and turn your Map into The best survival ever. If i will have more ideas I Will share it with you imideately.
Just Another Idea:
Add Builder Class System.
Ways of Builder training. 1) Fighter <= Standart builder => 2)Deffender
3)Рhysician
1) Can construct powerful towers with Big attack damage and Attack rate. Can use Mines and bombs with detonation. His magical towers can defeat all enemies with their's rockets.
Team Role: Damager
2)Deffender. His walls just undestructible! Can construct walls with big Hp amount and Armor. Walls can has abilities such as spiked walls (Just like buildings of orks)
Team Role: Tanker
3) This builder has small ranged attack. His cup of tea - Magic. can build Aura towers, Towers with Fighting Magic And Recovery ability.
Team Role: Support
And make green and Stinky zombies =))
http://www.youtube.com/watch?v=sZcREFrrA7k
i suggest you to Watch this.
Waiting for your answer.