I would keep any swords to one side or the other if you want one, behind the wood rather than in front. Keep the L. repens in a bush so it serves as a midground plant. Which pennywort, the big stuff that wants to be a floater or the little stuff that looks so great as a ground cover? Either is terrific.
Remember negative space is important too, there is no need to completely fill the back of the tank. A 55 gallon tank is so shallow front to back it probably will look deeper if you do leave a bit of the back showing.
The middle foreground is going to be bare? What about adding to that pile of rocks on the left to make a bank that ends just right of center and putting sand for a river bed? The rocks would look great with little Anubias or moss growing between them. The hairgrass would slowly invade the sand which could look very interesting I think. Of course you could put in a plastic strip to slow it down with more rocks hiding the plastic.
I get annoyed as it takes me a while to sketch the tank then I change my mind and it all gets really messy on paper. You could try using Paint, at least you get color and can properly draw the size of the tank pretty easily. I even copy and paste actual photos of my elements to a sketch of the tank. That is pretty fun to do. I am too impatient to figure this program out but I see amazing scapes made by others, might spend some time playing with it anyway.