Lightning is most likely gonna be in KHIII. Sorry, but it's just gonna happen eventually, Nomura likes her a lot and there are more fans who want her in it than don't. A more likely female character who will absolutely not be in the game is Luna. And it's because Nomura did not design her and probably didn't even create her.
As for me, I don't want Noctis and gang in it. The reason isn't because I don't want this game associated with FFXV, it's just that I think at this point, both games need to stand on their own. I think at this point, having Noctis in KHIII would just be distracting.
If we're sticking to FF elements, I also think the only character from pre-FFVII who should show up is Kain (and possibly Cecil and Golbez)and that's only because Nomura likes him too. Too many people complained about how Setzer and Vivi were handled and even though he did redesigns for the characters for Dissidia, I couldn't see what characters like Firion would contribute to the story other than fanservice or something. I think with Kain, it'd be more interesting to see him act as a foil for Riku, since Riku took after him a bit in the first game.