I think you may be right. I've been three times and can't get enough. I agree with Yan that Mt Aso is worth the trip. Also check Nabegatake falls and Takachihio Gorge. If you prefer to be away from the crowds (not that there are many on Kyushu) there are also many islands worth exploring, like Tanegashima and Yakushima to the south, and the Goto Islands to the west.

Have fun planning. I'll be headed back myself next autumn, although to the other end, to see more of Hokkaido.