He wasn't just a dream, there lies the problem.
If he had been just a dream, no real representation of someone in Zanarkand, then why did Jecht, who was a real Zanarkand person recognized him as his son/representation of his son? If he had been no one in Zanarkand, Jecht wouldn't had any reason to recognize him.
Shuyin being an afterthought makes really no sense, they had to explain how Tidus managed to fit in 1000 year old Zanarkand, which they totally left out of FFX. Vegnagun on the other hand, now that's an afterthought, as there is no real reference to it on FFX, nor to any of the main players in both the Youth League and New Yevon. The trio of Paine, Nooj and Baramel were also complete afterthoughts.
But I can also find other correlations between incomplete parts of FFX and content in FFX-2, but those two that I pointd out to you, are the most obvious ones.
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