I have hardy banana (musa basjoo) (9') in the back of my bed around the pond, with the hardy hibiscus in front of it (5').

You could also use cannas if you wanted something shorter, but still tropical looking. Although like me in zone 5, you'd have to dig them, and the banana, well you only have to mulch it heavily to make it come back in zone 5.
Some of the taller coreopsis would look good and fill in and bloom while your hibiscus is growing. I also have a nice perennial, red dragon persicara which is a good filler, not invasive. Or reseeding annual purple perilla, which gets to be around 2 1/2 feet tall in ideal conditions, and drops seed and returns each year.