I would suggest just a couple of things. You have a lot of leafy stuff in there now. So break that up a little using accent plants to add more interest. Something tall and vertical (a hand full of dwarf onions, maybe).. something showy with broad or strappy leaves.. (some apons or crinums). Do this with something that also adds reddish or darker colors, and it's even more dramatic.. like that dwarf lotus plant dafferspup suggested or a red barclaya. Add in some (visible!) driftwood or large rocks would help too.