Just dabbling in GIMP, following some YouTube videos and seeing what kind of results they yield. Here's a simple white marble wall:
Along with this, I will try to make some different wall features, such as pillars, alcoves, windows, doors and door frames, etc. Maybe some decorative stuff?
Then I'll play with some grunge brushes, adding cracks and imperfections, stains etc.
I think it would be cool to come up with some base styles from which additional kitbashing could be done.