Oh man, the fun I could have with this thread. My creative mind and lack of caring about other people will make some great stuff here.
Final Fantasy X(-2) is the past of Final Fantasy VII, as proved by Shinra when he discovers the energy in the planet. Thus, in the future of FFVII, when Cloud dies in the Northern Crater, he clings to life, because he feels he hasn't dealt with everything he should have, much like how Seymour and Auron did in the past.
And after he defeats Sephiroth, he can't go on to the Life Stream because there's no Summoners to help him move on.
And then, Sephiroth, using another person, comes back to a half life to try and bring Cloud back to the Life Stream, because Sephiroth, after talking with Aeris, has made a revelation and now feels the need to help Cloud back to the Life Stream.
(Yeah, I'm just pullin this outa my ass. I don't really believe this.)
Summon Materia isn't made like other Materia. Blank, non-magical materia is occasionally formed, and when a powerful individual or group (KOTR) die near this blank materia, instead of going to the Life Stream, the blank materia absorbs the persons soul, and creates a summon.
(THis would be pretty cool twist to FFVII)
FF in General:
What if all the final fantasy are just part of one long timeline that is constantly changing because of earth shattering events that rearange the planet it self, and the games are all just important events in the timeline of the planet whenever it's threated by some strong, evil individual?
What if it's all fake, and we're just dreaming? Or we're in the Matrix! And the Matrix movies are the way the people outside of the Matrix are trying to tell us what we're in? Or even worse! George Lucas is planing on making another Star Wars movie based on what happens before the original trilogy!?
Well, take that into what ever direction you feel like, cause I'm most likely going to laugh manically about it.