I don't think it ruins FFX, but how can you deny that Tidus coming back and all that happy joyful stuff cheapens how great and sad the ending to FFX was. Like now you know, oh hey he disappears but it's okay, he comes back and all is well. Him disappearing after holding Yuna like that is really sad and just awesome but if you know he's coming back it takes away from it, for me anyways.
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ExTha