The image is pretty small so hard to get a good look but here is what I would suggest-
- If its carved out of rock, make some decisions about what specific rock so you can get some texture information. It needs variation in color and texture
- You can’t carve perfectly straight lines in rock so it should all be damaged and bumpy and lumpy and just feel like it took effort to create this place
- Feels like theres a lot of light sources, obviously the one from the stairs is the “entrance” to the place but whats the white bar in front of the bed and to the side of it and coming from the doorway?
- If a 6ft person fits on that bed, those stairs are pretty big. So the beds are too small - unless this is a place for children which could work but then needs additional elements to sell that
- The ceiling alcove thing not being centered bugs me
I would start with making some concepts if you can, figure out some of those imperfections and structures and different light sources. Since the piece is tranquil and basic like you said, the idea and the lighting and the feel of it should do the bulk of the work. Needs really really good lighting. This is gonna be a hard task, but the idea is unique!*
Here is an image for you*http://static.boredpanda.com/blog/wp-content/uploads/2014/11/carved-cave-5.jpg
Note all the incredible detail and variation and the feeling that its carve and eroded, it has imperfection. You don’t have to make it look like this of course, theres tons of images out there, but it should help get you started!