Are you going to colour it eventually? if so that'll sove your problem real quick, just toss in a place marker for now

I like the sandworm idea, that's great. maybe a few cactuses and such or boulders near the southern boundary to help define it. Some dunes perhap? Though that might look too much like regular hills/mountains.. but if you labelled them (White Dunes or summat) that would help the illusion.. Once you name the region it'll help too. I think it's coming along fine.