OR the 3 oracles and/or the 3 goddesses. The goddesses are never expanded upon besides the fact that they made the world and they left behind the triforce each representing one part.
The ORACLES however need to definitely be expanded upon. Three women with the same names as the goddesses who all have special powers, and no one cares? Hmm...
I personally would love to see the return of Dark Link; not as a main villain, but possibly as a boss in a temple/dungeon. Although Dark Link did appear in Spirit Tracks, it's been awhile since he has appeared in a home console Zelda title (Twilight Princess only showed brief cut scenes).
Seeing the Old Man from the original LOZ would also be pretty awesome!