Like many have mentioned, your bio load isn't really the issue, it's more about space for all the fish. Think of it as too many dogs put up in a pen together...eventually something isn't going to work out. I'd start thinking in the realm of inverts instead of fish. There are lots of cool crabs out there that you could add, and you may be able to get away with a sexy shrimp or two, which are my personal favorites. A tuxedo urchin could also be a cool addition.
Why not add some flowing coral? Euphyllia, bubble tips, etc.? That'll add color and movement that you're looking at without causing a stressful tank environment.