Personally...I don't think Tidus was a dream...I just think he was dead.... like Auron, Seymour, and other people as well.... at least thats what I think.... but another thing is the he entered the farplane... so that doesn't work huh.... another thing that I can think of is.... The Toxin... from sin made everyone believe Tidus was a person there... there's several ways of the plot...