I'm pretty happy with the FF characters being relegated to minor fan service as opposed to major stars in the series. The main storyline and cast are far more interesting than seeing what Squall, Vivi, and Selphie are up to. I'm happy they at least allowed you to use Auron so I guess I can't argue for more FF party members but I don't want them to have a larger role in the plot than they already have.

Still, I would love to see Gabranth, Shadow, or Kain appear as the third FF character Hades sends after Hercules.

I'm also up for another FF super boss that isn't Sephiroth. Though Kefka would be amusing, it might be more badass to have Chaos appear with his Dissidia abilities...

Also, who is to say that they should only be KH3? BBS is still on the horizon and no FF cameos have been spotted yet.