Hello, person reading this who is potentially an nZombies developer, here are some suggestions that you may want to take into account.
1. More bosses/special zombies - Examples: Brutus from MOTD, Space Monkeys from Ascension, Margwa and Parasites from SOE/Revelations, Avogadro from Tranzit/AO, Jumping Jacks from Die Rise, etc.
2. Include a custom knife and/or grenade skin for the knife and grenade in the content packs, or leave a link to the custom models in the required collection.
3. Make different sized barriers for wider gaps.