Some milfoil or parrots feather in the back would be nice and they grow rapidly and tall! I really like the patch of grass off to the left. What is it?
Am I weird that I don't particularly like sand? I think gravel looks more natural in planted setups. Not the colored gaudy stuff but black with earth tones in it. Maybe its just me.
What I would do is take out the huge piece of slate, break it into smaller pieces. Find a tall/long root like driftwood; one that will reach from bottom substrate to surface of the tank and place it where the slate was and taking the driftwood(bogwood would be better) plant/erect it like I said where the slate was, then use the broken slate vertically intermingling with the driftwood piece buried in the substrate partially...like intermittent(not same height/abstract)fins. Have some moss on the wood and dwarf hairgrass to intermingle with both the wood and the slate. Add some micro sword surrounding the wood and some Anubis behind that entire piece. I would also use a couple piece of small twisted/frayed bark on the floor(not to big)that create additional focal points jutting at an angle from some of the detached hair grass.
The imagination is the limit; there is no limit then! Create, create and create some more....giving natural life to a tank is beneficial, fun and wholesome to the inhabitants and it's aquarist!!
Now I have 1 x Gold Severum, 1 x Blue Acara, 1 x Rusty Plec, 2 x Guppies, 1 x Danio, 1 x Sword tail, 4 x Tetra (cannot remember the type though) will upload a pic for identification