I have always known that Kuu loves her ballads to death. She has always enjoyed singing those and that's also why I became a fan after hearing you, Promise and Kiseki. Those took my soul and never let it go again. Even to this day I get most excited when she releases ballads. Upbeat songs are great too, but for me Kuu's ballads are the ones that charms me the best.
Also, one of my all time favorite J-Pop albums is full of ballads. It's BoA's VALENTI. I have always loved it and I think songs in that style would fit Kuu very well.