I think it is always more interesting to try to establish a sense of a large scale to a smaller environment. If you place large objects in a small tank it will make the tank look even smaller. Use some thin, long pieces of wood and small or narrow leaf plants to make the tank area look bigger than it is. Of course smaller fish would be more appropriate to this size tank size as well. I agree with altering the symmetry of your arrangement. Try be more natural with your arrangement by grouping elements together to establish a sense of visual flow from one area to the next. Although you can attach some plants to the wood, providing a greater area of plantable substrate will allow you more choices in plants.
Good luck!