Well there are a lot of things you could put in here that would go along with the variety you have already.
Can we get some specs on the tank. Size, lighting, CO2, maybe prams if you got them ;p
Right off the bat I would say something like Ludwigia Repens in the back right would be nice, or Brazillian Pennywort (Hydrocotyle Leucocephala), both of witch I have in my tank and trust me, they grow like crazy and have to be trimmed ;p
As for the front right, maybe a moss ball or a rock/wood with some java moss would be cool. If not I might think about throwing in something small like a small bush of baby tears (not baby dwarf tears, commonly known as HC).
Be better, as I said if we knew what you are running in there for a better suggestion ;p