Underground hazards like gas build up etc. are fine so long as their is an ongoing cost to maintaining them, not just 'build an air filtration system and your done' and I mean besides using fuel to keep motors running and the like.Things like replacement filters, durability on components so things break down and need new parts thus requiring looting trips to obtain more etc.
The challenge needs to be vaguely comparable to those faced by an above ground base.
I love building underground but that's a recipe for boredom currently, make it challenging, the current system allows me to make an underground base that is fully self sufficient (food, water, gas etc.) in complete safety with the odd bit of daytime looting.
Include a toggle off option in the same way as weather survival for those that don't want the added challenge.
That's how I see it, as you breathe out you build up toxic air and you have to have a ventilation system (which requires specific equipment) that requires electricity (generator, gas, wiring) to pump out the bad and bring in the good, filters (metal, ores to make them, cotton to be part of the filter, plastic for the frame) to help keep it clean. Being safe should have some cost to it, not a get out of jail free card. But that option should be there for those who want to live underground and like to come out to loot, hunt and even fight zombies at various times of their choosing.
We all get up in arms about the zombies respawning in a POI that is taken over as a base or as we loot it, yet it seems it is okay to state that zombies should spawn in a underground base to force the players to be above ground.
Tit for tat, if you don't like zombies spawning in your POI base, don't go stating that zombies should spawn in a underground base to force the player to be on the surface.
It keeps coming back to others wanting to tell someone else how they should play the game. If the developers plan for it that is different, but for Bossy with the my way or the highway attitude, I go take a long walk off a short pier, preferable into shark infested water loaded with chum.
Make it a option, toggle to have it on or off; set for extremely high game level so the causal player, the ones who have a life outside of 7 days, can still play the game.