I'm finally caught up on everything, and wow A20 is looking amazing!!
I know you guys really want extremely complicated to implement feature suggestions, so if you guys are really really bored while working on terrain generation, expanding on the underground generation at some point would be really cool. A cave system generator might be a bit too taxing, but a "spawner nest" that spawns near areas with too much heat on the heatmap and spams out zombies forcing you to go clear the nest would be pretty neat. There could be quests for it too, where a trader sends you to go clear a tunnel complex full of strong zombies to reach the "core"
Sure, zombies spawning out of a nest may not make sense to zombie fanatics, but who cares about the babies crying about muh lore breaking burnt zombies and zombie spawners.