Both Celtic- and Tchaikovsky-inspired scores would be amazing, though I probably lean a little closer to Celtic for the movie overall. But Tchaikovsky would be excellent for the scenes at Cair Paravel and the Snow Dance for sure!
Adam Young is a talented artist with some very creative and enjoyable works, and I'd be super happy to hear something by him in the credits of any of the remaining CoN films. I'm just not sure his score work is developed enough yet for a full-length movie. He's sure to keep improving though, and if the process of bringing SC to theaters takes a few more years maybe he'll be ready!
My personal favorite idea Future Narnian's suggestion of Patrick Doyle. His score for The Little Princess (1995 adaptation) really caught my attention, and since then he's also done Brave and Cinderella, as well as several Shakespearean adaptations. His music has a fantastical quality without being over-the-top or silly-sounding. The Little Princess has some beautiful small-scale choral (? vocal?) pieces that sound almost intimate instead of grandiose.
