There are only a few missions you have to complete per chapter to complete the story. And shouldn't this be in FFX-2 forum? In there, I found ALOT of problems with the game, but only you can form YOUR opinion.

Missions required

Ch1: Luca (concert), Gagazet, Besaid, Kilika
Ch2: Bevelle/Mushroom Rock, Bikanel, Gagazet, Djose, Bevelle
Ch3: Luca, Calm Lands, Besaid, Djose Temple
Ch4: Commspheres, nearly all of them optional, Moonflow, Thunder Plains
Ch5: You can fight Vegnagun right away

SquareEnix makes sequels/spinoffs to games to make money. They make actual continuations of the series to tell the story. In those cases, money is a lucky byproduct. And Yuna's personality change would not have been as radical as it was, or it would have had a completely different result on Yuna.