If Yuna going on her own adventure in a sexy new outfit seems a little jarring, perhaps it's because you haven't seen the true ending of FFX. See, after the game was released in the US, with a few new additions over the original Japanese version, our version was then re-released in Japan as Final Fantasy X International with even more new stuff--including a longer ending movie. Below is a summary of that movie, which clearly seems to have been a set-up for FFX-2.
Two years have passed, and Yuna, Wakka, and Lulu live on Besaid island. Yuna's been practicing holding her breath (she's up to 2 minutes, 41 sec.). Wakka (now sporting a pudgy belly) got together with Lulu and they're going to have a baby. Yuna describes this new era of peace as "The Eternal Calm".
Since the defeat of of Sin and the downfall of the Yevon religion, new factions have sprung up to compete all across Spira, groups like "New Yevon" and the "Young Persons' Alliance." Each is trying to get Yuna's endorcement, since she is revered by the people. However, Yuna is careful to not to be "used" by anyone, so she repeatedly refuses to meet with these groups. She spends most of her time meeting common folk who come to the area in droves to get a glimpse of her.
One day, Kimahri (who now teaches Ronso children up on Mt. Gagazet) finds a sphere in the mountains. Upon seeing the vision inside of it, he gives it to Rikku to take to Yuna. With the collapse of Yevon, machina use is becoming widespread all across Spira, and Rikku spends her time teaching people how to use machines.
Rikku arrives at Besaid and shows the sphere to Yuna: it contains an image of Tidus, trapped in a cage, clamoring to get out. The image is fuzzy; it has an unterwater look. He's yelling, asking why he was arrested, saying that he had to use the machina to save the Summoner. He's demanding to be let out, and to see Yuna.
Yuna is stunned. Wakka thinks they should "look into it", without rushing off. Yuna's time is booked for three months, and people will be disappointed if she skips town. Rikku is furious with Wakka, saying that "Yunie" should finally put her own happiness first. Remembering words that Tidus once said to her, Yuna decides to go. Since Yuna is so easily recognized, Rikku gives her some new clothes so that she can "go undercover", and they set off. Cue Final Fantasy X-2!