Sheik only appeared in Ocarina of Time if I'm not mistaken... I don't see why Ghirahim can't get in it seems like most people like him
Correct, Sheik is a one-shot like Ghirahim, but the case is a bit different for her. See, she is the same character as Zelda, so her inclusion added some flavor to Zelda's character, and a new flavor to the series in general, since it's the first transformation character we've seen. Sheik has much more going for her than just being a one-shot.
Ghirahim isn't much different than Midna & Wolf Link. That's a better comparison.
Which is why Impa replacing Sheik is so awesome right? ...Right??
As interesting as that might be, I doubt it will happen, because it will detract from Zelda's unique character. Plus, Sheik has been in the last two games of the series. Her cut would be unwelcome unless it were for a truly good reason, and Imps taking her place is
not a good reason.
I actually expect Young or Toon Link, and unless Ganondorf appears in a new game, Toon Ganondorf(two swords).
You can bet on some form of Child Link to appear (Ton Link/Young Link). Either one could apear, leaving the other one behind. I'd place my bets on Toon Link though.
Toon Ganondorf... I wouldn't expect him at all because 1) The current Ganondorf needs some long-awaited tweaking to his moveset 2) Toon Zelda would be the next Toon character, and 3) Toon Link's games don't seem to be producing anything new at the moment.