I do think you're right in terms of the time difference. It would make sense to have Tidus and a descendant of Dream Shuyin. And it's true that they don't look exactly a like. But to me, Tidus kind of looks like a younger version of Shuyin. I mean, look at Yuna's difference between X and X-2 (a two year time difference):
Attachment 1886
Attachment 1887
Her face changed quite a bit within two years. I would imagine Shuyin is a few years older than Tidus....so to me it's not that much of a stretch to believe Tidus' face could change a bit between age 17 and whatever Shuyin's age was when he died (my guess: between 19 and 24).
My problem with the descendant theory lies in the fact that not only do they look alike, they also have the same voice (same voice actors. lol), same overdrives, similar stances in battle and mannerisms, even using the same catchphrase in the Den of Woe (there can't be two idiots saying 'This is our story' unless they're somehow connected. lol). Another thing I forgot to mention in the Den of Woe:
Jump to 2:30
Look familiar?
I don't know. There are too many things about Shuyin that are meant to remind us of Tidus, even after the fact that Yuna discovers he's not Tidus. I can understand having similarities prior to finding out this fact, but what's the point in having us be remind of Tidus after we already figured out he's not Tidus? I see them as hints to the fact that Tidus is somewhat of an imperfect copy of Shuyin.
But I respect your theory. You may be right about the descendant theory. It does make a lot of sense, but to me it doesn't quite explain all the little details you see of Shuyin throughout the game.
One last thing I forgot to mention:
Shuyin was apparently a blitzer as well. Some fans dispute this and argue that there's not proof of it. But to me the fact that he uses a blitzball in the Terror of Zanarkand (and does the Sphere Shot. Which as we know from FFX is not an easy task to complete. When Tidus did the shot for Wakka and the Besaid Aurochs they were extremely impressed.) Plus there's the fact that his clothes look like they could be a blitzer uniform (a much older version of the Abes uniform from FFX)
But not only that, I found this in the FFX-2 Illustration Collection:
Attachment 1888Attachment 1889
I wanted to add that in to prove another similarity between Tidus and Shuyin.