I'm kinda struggling with the same problem. Here's my first project. It's still really dull and empty, but I plan on adding ventilation shafts etc. to fill some of the empty space near the roof in both the corridor and the bigger hall. I'm pretty proud of it so far, even though it looks nowhere near professionally made. It's not special, but it's mine.
I suggest browsing the workshop with care to find new stuff to fill the voids of your maps. I found a lot of neat stuff there and installed them just in case I need them at some point.
But I have an issue with lighting. That is as dark as it gets with no lights in the spawn corridor and I have no clue how to make it darker. I want it to be nearly pitch black so I can adjust the atmosphere with actual lights. Same in the big hall. I only have two blinking attention lights, one flickering roof light and one flickering smaller light. Once you press a button, all 3 roof lights turn on, the attention lights change color and the flickering lights stop flickering. But right now you can't even see the difference when they're on and off since it's so damn bright anyway.
Can I somehow make it darker? EDIT: User "Olby" told me to change the ambient light color from the root node and it worked. (Orignals: 46, 53, 82. I ended up putting 9, 11,16, which is a fifth of the original) Thanks Olby!
Oh and OP, your screenshot looks really good in my opinion.
As for the cubiness. Open up the AI tutorial and take a look at how they've made the map appear less like a cube. They've added support structures and stuff like that to make it look more realistic.