1. Midna - Arrogant as she may seem, she only cared about returning to rule over her people. If it included manipulating Link's integrity then it was done for the greater good. But I would want her to be shown again for the qualities that emerged later in the game. Midna was merely indignant about being deposed, but really did care for her people. She aspired to retake the throne in order to eliminate Zant's maniacal tyranny.
2. Happy Mask Salesman - I want to finally know who he is, where he came from originally, etc. It's to my belief that he's a Sheikah, though. They might be gifted with entering a shadow in any area and emerging into a destination of their choosing--sort of like shadow teleportation (they are the Shadow Tribe, after all). And this could explain how the HMS knew about Termina, and any other realm he appears in.
3. Sheik - Or some character very similar to him. Perhaps Zelda again in her male persona form, this time as your guide throughout the game. Concealing Sheik would be a problem, because most assistants need to go into hiding (excluding King of the Red Lions and the fairies), but Nintendo is very inventive when it comes to this. At any rate...Sheik is definitely a figure that merits a return, even as a cameo.
