I would have loved for Lulu to be a main character again...
But perhaps her maturity and calmness were the very reason she wasn't made a major character. After all, the game was about Yuna, Rikku and Paine being youthful and having wild adventures - Lulu would've been too sensible, and stopped them getting into mischief.

That was a nice quirk of X-2's plot: Spira's calm and peaceful after a thousand years, so people are finally free to have fun and actually live their lives without the constant threat of annihilation. Yuna, Rikku and Paine indulge in that, and really start to act their age again - until a similarly terrible threat emerges, and they have to rise to an all-new challenge. Lulu's been through a lot, so she's always prepared for something bad to happen. The Vegnagun crisis would've shocked her, but it would've had less of an emotional effect on her than on the others. She would've taken charge and known what the team should do, but her lack of impulsiveness might've held her back from approaching the problem in the most effective way.