Suggest things to put in super base

I carved out this huge area underground for a mega base project, but there is still massive amount of space even after I allocated rooms for things I already want. I thought of building a mob farm here, but I cant cos its in the underground layer.

So far, I can think of filling some of the space with various tree farms and herb farms. Any other suggestions of things I can add to fill up the space? Anything cool, fun or useful. Especially useful. My NPCs all have their own respectivr pylon villages alrdy (except hallow and mushroom)