The best plot would be if all the dead enemies and characters come back from the farplane. They can get out of the farplane just as Lord jyscal did, can't they?. He got out of the farplane and gave Yuna the sphere that tells that Seymour killed him. So if he got out of there i think everyone else can, even Yu Yevon with his armor ( Sin).

Or simply most of the story could be in the farplane cuz i guess it would be a bigger place in ffx3 than then previous games since in ffx it was a small room in Guadosalam and in x-2 it was a whole new place.
And if and not wrong all the Aeons in x-2 came from a hole connected to the farplane thus they came from the farplane, so everyone came get out of there and since the Aeons can and Sin is an armor made of an Aeon, Sin can get leave the farplane 2.