I think the best way to handle that game would be to give the player the option to (SPOILER)
1. Make Jecht the Final Aeon & follow the story as canon.
2. Make Auron the Final Aeon instead.
3. Not use the Final Aeon & have Braska follow the path that Yuna eventually did & they defeat Sin by force.
The last two options would retcon the whole series & the 2nd could lead to an interesting "what if" sequel, where Jecht has to recruit his son (who he hasn't seen in 10 yrs.) to defeat Auron/Sin.