Hmm that's a tough one. Are you looking for another bottom dweller other than the yellow watchman goby? You could instead of the YWG get a shrimp goby like a pink spotted, they get bigger and would probably fit your size tank better. I have both a pink spotted and a yellow watchman and the YWG is quite small actually, he is in a biocube now. The shrimp goby I have in my 125g and he seems to fit that tank better.
If you want some mid water active fish maybe some chromis? I have blue reef chromis and they are awesome, they are always schooling together.
Just a matter of what you like is all. Any other fish you might have your eye on?