Well you have bottom-dwellers already and plenty of schooling fish to fill out the middle so I'd get some honey gouramies (a trio) for the top and a trio of croaking gouramies or 3-5 sparkling gouramies to add some non-schooling interest to the middle and lower levels.
If you don't mind adding something bigger, I'd suggest a pair of pearl gouramies instead to fill out the top and act as a focul point.
On the other hand, for non-gourami fans, a school of myer's hatchetfish would fill out the top nicely or maybe a school of some kind of danio (though I'd personaly be all for the common zebras or leopards).
Also, a pair of rams would make a nice centerpiece for those who preffer cichlids or some small livebearers such as platies, guppies or endler's to add colorful highlights to the set-up.