Jon Landau, one of the producers on Avatar, and a producer on James Cameron's Battle Angel Alita film discussed the plot in a recent interview.
Battle Angel Alita is my favorite manga. It's basically the only manga I've read. Besides a couple of one of issues of things like Tenchi Muyo and Eat Man back in the late nineties, the only manga I've read is Alita. I've watched my fair share of anime but never really got bitten by the manga bug.
It was hinted and obvious from the start that they'd be focusing on Motorball for this film. It's the most accessible plot line of the series and would best translate into a straight action film. It's something I see Cameron having a hard time messing up.
Focusing on Motorball, I imagine it will be hard to move on to the rest of the story. Motorball is merely one of the settings for Alita's reawakening and remembering her powers. If they start the "series" with a Motorball movie, everyone will be thinking it's all about Motorball and any future installments may be distorted or never even be produced.
Oh well, I suppose it's not worth worrying over. Even one Alita movie, which I never dreamed I'd see, is a treat.