Personally, the creator of Ar Tonelico reminds me of myself. From the distinct presentation of himself, to his philosophy in designing games.
I think a spiritual successor would actually be a pretty decent idea. Something different, but at the same time, not so different. Personally, I feel like an Action RPG with fights choreographed to the Soundtrack would be pretty sweet. But I don't know if that would work quite right in Ar Tonelico, but for a new RPG it just might work.
Though I wouldn't go with the Battle System for Qoga, I'd personally try to develop something kind of like what was used for the original dotHack games on the PS2, but have a small menu for quick commands on the fly like in the Kingdom Hearts series, since I'd probably have the game pause the music when you go into the main command menu, that way battles can remain synchronized to the music, but that way you can't skip a tough part of the song by going into the main option menu.