A prequel would be a dream come true, but it wouldn't be on Braska's pilgrimage think bigger, go back in time to a 1000 years ago where everything started. Spira was alot different back then Sin destroyed alot of the land over a thousand years, it would be a completely different Spira with huge Machina cities, different modes of transport.

Spoiler Alert (probably minor better be safe though): You would also see the creation of Sin, the creation of the dream, the start of the Machina War, Yu Yevon, new aeons. It's possibile there were summoners before the machina war. You would probably see the creation of the Aeons in FFX, did the fayth really just willingly give their souls or were they just tricked? Aswell as Yu Yevon, you would see other characters such as Lady Yunalesca, Lord Zaon, Shuyin, Lenne. There is so much of Spira's history that wasn't fully explored it could easily make another game .