Just give the plants time to grow and fill in. It makes a world of difference in how the tank looks. The only thing I would think of it something small like a little Crypt Parva
http://www.aquariumplants.com/Cryptocoryne_Parva_p/cr91.htm right in that hole between the big cluter of rocks and in front of the next big rock over. Something little right in that triangle to accentuate the rocks. Then maybe something low and stemy like Staurogyne repens
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in the open area behind the rocks but in front of the driftwood. A small stem plant will look good in shape and texture with all the sword shaped plants. Really good start!
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