Ending Theories
Theory 1
Cloud will defeat the summon and eventually die from it and saving the world once again. The world will be saved and so does his despair. He seems to live just to exist but not really to really live (pretty confusing eh but I hope you get my point). Cloud has become such a pessimist ever since Aeris died at least that's what I think as one of the reasons he's like that >_<
Theory 2
Pretty much like the 1st theory however this time he goes with Aeris. Cloud dying in Tifa's arms and then Tifa saying something like "Go and meet her". Cloud dies finally and the ending shows Cloud reunited with Aeris. (If this happens I'm gonna be really sad for Tifa, such a brave and loving girl T_T) I guess I'd be happy for the two of them if ever they'll be together though =D
Theory 3
Cloud will save the world again and finally realizes that there is still hope after all. Cloud can be seen once again in Aeris' church seemingly talking with Aeris in thoughts and then he stands up then heads out. Outside the church he'd see a woman waiting. He then looks at Tifa, the one who has been always waiting for him and still she is. He realizes how blind he has been from all these years and you know what happens XD (i'd like this ending very much, I just hope this can happen, if not exactly hope it concerns Cloud and Tifa =D)