the list of characters that haven't sung yet is fairly small.
out of it, i'd say Luna, Big Macintosh, Discord, Cheerilee and Zecora have the highest possibilities.
i'm all for Luna. especially if it's a duet with Celestia
many are thinking Discord, because at Las Pegasus Unicon someone asked John de Lancie if he would be singing in Season Four and his response was "just remember you asked that question"