it would be cool if you found like glowing underground fungi, and weird creatures that have never seen anything that comes from the surface, but those are just ideas
We already have some sort of implicit lore around that. See Expanding the Underground
I think one thing I might do before the release is adding some more map extras to subway tunnels so we could have tunnels which are flooded (well, at least one overmap tile of them) or where trash has been disposed of.
Edit: One thing I’d also like to see would be bumblebee nests. We have giant bees, so we should also have giant flying pandas.