I'll say put some easter eggs in it. Here's some:
1. Legend of zelda easter egg: You can find a a sword called the master sword (It can 1 hit kill zombie's but it has like 10 charges)
2. TF2 easter egg: You can find a gun called: "The admin" and you can lower zombie's HP by headshoting them with it.
3. Sonic easter egg: Very rarely you can find a glove called: "Knuckle's glove's" It wont do much but its a mleee wep
(LAST ONE) 4. Minecraft easter egg: In some of the maps (If you want that is) You can make a minecraft themed area like in borderlands 2. If you break the drit, you find minecraft zombie's and steve killin them. He gives you a snipe gun called "Sniper riper" And then he leaves. The gun is just a normal sniper gun.
These are just what I think
