When the game was first released, there was a topic at another message board about a similar subject and how the game could have continued on a bad ending path in which Yuna decides to sacrifice one of her guardians.

Everyone discussed various ways that a Final Aeon could have been implemented in the game, and a dissecting storyline was widely regarded as the best method rather than providing only what if scenarios.

I believe that the dissecting storyline evolved into Yuna being able to choose from her core guardians. I recall users agreeing that Rikku would not have agreed to the process and would have left the party upon finding out that Yuna was willing to go through with it despite knowing everything was a lie, so Rikku was not able to become a Final Aeon no matter what. It was an interesting brainstorming session.