I think it's funny to see people call Impa a "one-off character" at the start of the thread (just now reading it all) like Sheik wasn't. Unfortunately, Sheik is less useful as a plot device/character in the Zelda series now, hence her cut from TP. They can't pull off that same trick again with her without inventing a new character like Tetra. In addition, Impa's height makes her fairly unique. She's like a freaking Amazon!
I could see Sakurai putting Sheik in as Zelda dressed like Impa or something, but I think he'd rather add a new character to the character select screen since he's already having trouble expanding the roster due to all of the balancing work that needs to be done. Plus, SS Zelda doesn't come off as a turns-into-ninja type. (Moreso than the others, but I wouldn't complain if she did.) The only way that I could work out him putting in Sheik Classic is if he intends to do DLC and clone Impa later. (Obviously, it wouldn't work the other way around because Zelda would have an empty move.) We know that he communicates with the devs and it really seems like they designed Impa to fill in for Sheik since the two already had character ties in the previous incarnation. I think they might even recommend it.
We'd all love to see both, but after the discouraging news about the minimalist character update, I hope you'll understand that I'm trying to see things from Sakurai's POV.
Edit: Didn't mean to cut that Pichu line out on you guys. I just figured it was less relevant since it was an early quote. I didn't think I'd get a few replies in minutes.