For the Sun I think Luneth from FFIIIDS would be a great choice, because he is very young, and innocent, but not naive as you stated. I think of celebration, too, because he was the first chosen by the crystals and the elders in his village state that they knew this day would come. Also, probably because the FMV of him riding on a chocobo is very memorable, and I know you said that would be a good idea of someone riding underneath the sun.

For the world, I think Bartz would be a good choice, as at the end, he has overcome the antagonist, restored the 4 crystals and subdued The Void (5 elements), and they do decide to go on another adventure in the ending. the ending scene really reminds me of that pause between adventures.

I think Ricard Highwind would be a good hanged man, he has a different perspective from others as the Dragoons were isolated from the rest of the world on their little island, living with Wyverns, and he was the last one. As with the latter part, he could have very well escaped his fate, it seems he simply wanted to see if he could survive a 1-on-1 showdown with the toughest S.O.B. (also the unknown), and he definately felt it was for a good cause.

Gramis seems perfect for The Emporer, although I think in the end he chose that Vayne's way was the correct path. Also, I think there may be a better one for Heirophant, i'm just not sure right now...

this is all very interesting, and i'm not sure if i'm doing this right b/c i don't know much about tarot cards, but definately keep us updated on this.