Although I admit it would be great if she indeed show up, I hope I'm not the only one here who feel there isn't a need to show her in KH3, or at least as an integral part of the story. I would already be content with a small cameo of her.
If I recall correctly, it was mentioned in the Journal in KH1 that Leon cast Squall aside because he failed to protect home world. Now that the two events are linked (losing his home world and him knowing Rinoa), I like to think that Leon lost Rinoa for good; that would give more depth and meaning to his aside of his former self, as it not only represents his inability to defend his home, but also the people he treasure the most.
Of course, a reunion would be all fine and dandy and I have nothing against that. This was just my opinion and preference.