Looks really, really nice!! I think that if you can find a nice background plant to hide some of the planters, it will be a wonderful looking set up. Always nice to see a fellow ripariumist!
I would suggest some root tabs and some Hygrophila Kompact? It would get nice and full without getting ridiculously tall and would not require constant pruning, unlike it's brothers and sisters in the Hygro "family" of plants. If you are looking for some contrast, a few stems of an Alternanthera species would also look great, and would grow emmersed from the aquarium helping things blend in well.
Really, though you have a ton of potential options though, as you probably well know. For root tabs, I have to recommend Devin's line of GroSoil.