I am putting my money on an actress from a princess movie, and going to be more specific, it's going to be a non-Caucasian Disney Princess actress/singer. Anika Noni Rose, Kelly Marie Tran, Auli'i Cravalho, maybe Lea Salonga or Irene Bedard, but their movies are probably too long ago.
If they do Frozen, I think they will do both Idina Menzel and Kirsten Bell together.
It's not necessary to have her to be an entertainer, but it makes a more interesting show if the person christening the ship can perform at the ceremony as well.