With a 30g tall, you will have the height for a deep sand bed. When I had the mantis in there I had a 6" sand bed. I think a colony of jawfish, with the tank mirroring a sand flat would be cool. With the height, and sand depth, you may also be able to keep a sea grass or two just to make it look more natural, You could also add a solitary sand coral like a fungia, etc.
I think it would look really cool.