On my larger bases, I usually add some 'larger' rocks to the base before I add sand (for texture), pulling a random example off Google (spoilered for large image):
Then fill in the rest of the base with your standard method. I add little tufts of static grass afterwards too but you've got more of a barren-rocky thing going on.
Other options would be other kinds of debris from leftover parts of other kits (i.e. dead space marine's helmet / shoulderpad). This can be tricky from the viewpoint of trying to make it look like it's sunk into the texture of the base (partly covered, not lying completely flat) rather than obviously super-glued on top. For me this means cutting / filing down part of the surface at an angle until you're happy with it (dangerous method for fingers though).
GW also do basing kits of their own, which can provide some inspiration:
https://www.games-workshop.com/en-GB/Painting-Modelling?N=3815391097+2202671284&Nr=AND%28sku.siteId%3AGB_gw%2Cproduct.locale%3Aen_GB_gw%29&Nrs=collection%28%29%2Frecord%5Bproduct.startDate+%3C%3D+1520160600000+and+product.endDate+%3E%3D+1520160600000%5D