I honestly never used those features much outside of the first few times I played those maps. They were a nuisance in my opinion, and portals are much more convenient and practical. And gumballs are overpowered but I think the fact that you can't get that many from the randomness of the factory balances everything out.
Anyways, here's my gum ideas:
Infradead Sight (Common, lasts two rounds): Allows players to see zombies through walls for two rounds as red outlines (like the oracle in Ghosts). Great for planing escape routes and choosing the safer path.
Sanctuary (Rare, one activation): When activated, when the current round ends as the last zombie dies, the next round won't start until one more minute, giving players who accidentally ended the round early to do stuff.
Cash Back (Common, lasts until the end of the current round): Lasts for one round, and everything bought gives the players a fraction of the points back.
Truly Upgraded (Ultra-Rare, two activations): When activated, the gun in your hand can now get two Pack-a-Punch perks now, stacked on top of each other.