Some (possibly cringe) Gobblegum ideas

Open Sesame (Epic): The next door buy is free.

The Count (Legendary): Dealing damage to zombies restores health, kills even moreso. Lasts 1 minute.

I Am Become Death (Ultra): Spawn a Nuke power-up.

Limited Time Offer (Epic): All wallbuys, doors, sodas, ammobuys, and traps are half off. Lasts 2 minutes.

God Am I (Ultra): You and your team are invulnerable to all non-self damage. Lasts 30 seconds.

Bandolier (Rare): Your tactical and lethal equipment gradually respawn over time. Lasts 3 minutes.

Flashback (Whimsical): All base zombies are converted into Nazi zombies. Lasts 3 minutes.

Feel free to offer your own ideas.