I love the idea of a prequel game! I do agree, however, that it probably would flow better as a CGI movie like Advent Children. The story involved in FFX has so much depth and history that I think it would require several movies. One for Braska's pilgrimage, one for the machina wars 1000 years earlier,etc. Personally, I'd like loads, because I'm greedy like that.
I write fanfiction, and at the moment I'm currently writing a FFX Novelisation. The notion of writing an account of Braska's journey has often appealed to me. I might just do it