How about small/cool livestock that's not a fish? You could get a shrimp for your goby. Or a porcelain crab.
I have a pair of transparent cave gobies - liveaquaria has them sometimes. They're strictly bottom dwellers and dig a bit, but fit the bill of "interesting fish you don't see unless you look for them." Also have a starry blenny that's incredibly secretive but really cool when you do actually see him - that might be a better choice since you already have the goby to dig in the sand.
I'm interested in this niche, too so I'll be reading what other suggest. It's a hard one to fill IME - some fish dig too much, others eat desirable microfauna, others spend their days sitting on corals and killing them... I think you've got a good start with your hawkfish and goby though.