You have quite a bit of color in there already!
How about some red wag swordtails or some platies. I think something in the red/orange colors might make your tank really look good. If you don't want to go for livebearers, maybe some tetras with red/orange in them, like bleeding hearts.
You could also liven it up with something very active like danios or rasboras, which tend to be a lot faster moving than the gouramis and tetras. You might find that adds more interest to the tank than something colorful.