Yuna is still Yuna, she's just had a lot of Rikku rubbed off on her. FFX marks the end of an era in its world, so of course there isn't much to expand on, however, FFX-2 delves into some of the world's history. The chapters thing was pretty dumb. I was fine playing with three girls as characters, and found the dresssphere system logical and consistant with the lore, as well as making much more sense as a way to swap out different capabilities during a fight than having seven characters who all fail if three of them get knocked out.