dragon gobies get to like 12" maybe more. you could do some of the smaller gobies, like knight gobies or bumblebee gobies. even butis butis.
you could get the smaller species of mudskipper in there, or a figure 8 puffer.
if you do go for the dragon goby, an upgrade will be on the cards. biger tanks have more options, and brackish fish generally get quite big. archers, scats, monos, freshwater moray eels, columbian shark catfish, and the brackish datnoids, are all capable of growing over 12". and that includes the dragon goby...