I honestly don't think Noctis is likely to show up in KHIII. I could be wrong, but I wouldn't get my hopes up.
I think Sazh might have a shot too. Much like Kain and Lightning, Sazh is a character Nomura is fond of. Rinoa is the next likely candidate for FFVIII, maybe Laguna. He was intended to be in Birth by Sleep originally, after all. And we can definitely look to see FFVII have a new representative. I'm guessing it might be Vincent, another character intended to be in one of the games.