We solved it. Here's the solution:

First we turned on the power, which is buildable via parts found around the map.

Then we did the soul boxes, which are these crates with fruit in them found at various points around the map. Just kill zombies close to them, and you'll see the blue souls getting absorbed. When they're done, you get either a max ammo or a perk.

These opened John Candy's tomb, which is located in the underground part of the map. There's a skeleton in this new room, which serves as another soul box. Once it's complete, you get Ray Gun 3s for everybody.

After this, we had to find the Riddler's skeleton. This involved finding 3 skeletal parts around the map: a skull, a skeletal hand, and a pile of bones. Just press F on them when you find them. There aren't any interact prompts on most of the easter egg things.

This opens the gate near the Riddler poster, which has a staircase leading up to a snowy outside area with Pack a Punch. Buy the debris to get to a gate with a sign, which says "Escape From The Tomb" in Norwegian. This indicates that you need to go around the map and interact with the "Escape From The Tomb" posters. We're not sure what worked. We pressed F, knifed, and shot them with pack-a-punched weapons.

This opened the gate in the outside area near Pack-a-Punch. Before you go down the stairs, prepare for a boss fight - it will spawn a ton of Brutus enemies for you to kill.

After you've defeated them, a buyable ending of 50,000 will be available.

Also you gotta be listening to "God Bless America - And All The Beautiful Women In It" by Lana Del Rey the entire time. Or at least, we were.