goin through a lot of caves in ffxiv i never felt the "creep".
They look nice but are pretty similar and light everywhere.

I would welcome places that are dark (also on the map, so you cant find your way easily) so you don't really know where you are, and can barely see something unless its "too late".

This way you could add a bit more "dangerous", encounters, bat swarm sounds and sometimes something creepy running away from you to add to the mood.

To see something in most of the dark parts you HAVE to have a torch or a light spell on you so you can see something at all. - ofc the torches can burn out . (=> crafting).

these places could be added in a lot of the existing cave areas to expand them, leading to a lower level, even more humid area with harder mobs, crazy aggroing stuff everywhere, and perhaps some dark temples/ruins to explore down there. Long forgotton culture. Or as many mentioned : TONBERRIES !!!! , Sahagin type mobs, snake type mobs. And harder NMS.

Quests could be added into these areas to offer some exploring challenge and fighting opportunities.

Going there with some friends wouldnt only make sense it would be a life saver. so a nice opportuniy for ls, groups, companies to add dangerous questing. (hunting down some kinda sea horror in the lowest level of one of the limsa dungeons that keeps killing the sailers)

Perhaps even an heremit npc down there offering some additional quests or something.

Rare gather spots could be found down there as well as some dark-ponds for fishers or mushrooms for gatherers.... well just my thoughts of caves being more dangerous instead of just differnet textures.