Welcome InvertedSilence!
Personally, I'd like to see Cassima and Rosella's themes from KQ6 and KQ4 (respectively) somewhere in the game. Even if it's just a snippet. I really thought they were very beautiful.
And, while the version of Rosella's theme for KQ9 that's listed doesn't give a good example of this, in the theme, I added just a hint of some of her theme from KQ4. While I'm not the official composer, I wrote some "possible" music for the game (some of which is on this website). I'm a huge fan of placing little snippets of past music into things - especially pieces that people will notice and make them smile as they remember playing those past games.
Heh, listen to me - going off as if I do this all the time. I am no professional composer and one thing Cesar is VERY big on is professional sounding music. So no matter what the music is, I'm sure it will be lush and fantastic!