Planting something at the top also sounds interesting, what did you have in mind for that?
The path seems to peak/end right center.
If you want the illusion of depth, something shorter then on the sides and front, which is a challange. Maybe S. repens, hydrocolite Japan, a single blyxa. Blah, me not liking those choices. For a focal point, a single AR mini. Dearf water lily might work. E. tropica aka Rosette sword. Gee