If you have 2 discus in the tank, and they are happy being with each other don't add more to the tank.
It would help to see what the tank currently looks like, so we can provide suggestions that can make use of the stuff you already have.
basically all you need is wood, some plants, and some dither fish like rummy noses seeing as you already have the other equipment. The tanks in the AGA contests are not even worth looking at as those tanks need far too much maintenance, usually daily.
what sort of light does the tank have? (need the K spectrum).