She's not a one off, has an important recent role / revival in the series (not many characters get this in the series), she's been there from the start of the series as well, and she is the only character to legitimately replace Sheik for. I say her chances are good, as far as Zelda newcomers go.
It also helps that Sheik in fact is already in the game, for now. It helps them to base Impa upon, as there are still some (normal) moves needed for inspiration, and Sheik's moves where all made up.
Ghirahim aka Queerahim would only open more oppertunities for Impa to be in, starring in the same game and all. Them being sort of rivals also helps with this.
Anyway. It's really doubtful they will leave Zelda with its characters like this. Sheik also seemed to me as likely to get removed pre-Brawl, and it seemed her inclusion was pretty much thanks to having a scrapped Twilight Princess design ready to get used (along with the obvious other designs as Link, Zelda and Gdorf). But seeing as there is no SS Sheik design, I think the Smash team will be prompted to use Impa instead.
That's unless Sheik returned in the next Zelda game though. Which I feel is highly unlikely.
So yes. Can we have Impa now?