I haven't played this, but there are evidently a few standout issues so a few suggestions for you:
It isn't enough that the lighting is realistic if it is hard to play in! How about some lanterns on the walls, they could cast some interesting shadows too? Or you could have crystals in the rocks that glow.
Caves aren't square, they are often carved by water or ice and such things do not carve out squares. To break things up a little how about (if you haven't already) adding some stalecnites and stalectites - I'm pretty sure I spelled those wrong but I mean those long pillars of rock from the ground, and the hanging rock from the cieling. Atmospheric too.
If you want to see a cave with Lava done expertly, check out Ancient Sith Homeworld for JO by the highly talented mapper SunBurN, it might give ya some ideas.
http://www.pcgamemods.com/1348/)