Looking nice, it certainly does need something in the middle to fill in the pare space. If not some milfoil (as lovely a plant as it it) you could get some nice twisty goldvine (or some other interesting timber) and either attach a type of java fern (needle leaf is quite nice and slower growing) or some mosses to it. Just remember to not place it dead centre of the tank.
I like your "crazy paving" that you have done in the tank, I can just imagine the shrimp skipping along it singing in high pitched little voices " We're off to see the Wizard" LOL.