Likelihood of appearing: Meh, why not?
Following the same logic we used for Peach, it also seems possible that Toad could be the unlockable. His play style conceivably fits, plus he and his lookalike brothers appear in the title already as NPCs, so not much work needed to design his playable form. We just hope it isn’t him, as Toad has perhaps the most annoying voice in all of gaming. With how talkative Mario was in the first Galaxy, the chance of hearing Toad speak that often makes us want to cancel our pre-order.