I personnally think they will make real mash-ups of Ayu songs (vocals?) + famous pieces of classical music.
If they intended to mash-up 2 Ayu songs together, they wouldn't have used the term "classical", but "classic" instead. There's a huge difference here.
Classical music is what we can hear in MY STORY Classical or A Classical.
Classic(s) is songs like evolution, M, Voyage, Trauma.... "Classic" in Ayu's terms of course.
Here's a video from Adooph (youtube-based remixer), that can be an hint of what that could sound like (I hope so) :