I have recently been reading through the Manga, and while I rather like it, there are many issues with it (Not the least the retcon ending of the original series, the fact that the second series seems like a complete reboot at times, and the convenient 'forgetting' of characters when the author gets bored of them)
Despite these faults it is a fun series, but increasingly it is very complicated. I'm not sure how well it would translate to film - the length is an issue, but the complexity too (just look at LOTR, which was rather badly garbled and is still probably the best film adaption) as much as I like 'most' of cameron's films - The Abyss being one of my fave films (let us never speak of the 'sinking ship' I feel he does reduce things down to a thin layered allegory (e.g, Avatar = native americans, Abyss = human capacity for violence etc) In some ways Alita would fit with that - at the bare bones it is basically asking us 'what makes us human' but I fear it would end up a rather garbled mess.
A new OVA series would be my option, but a director is problematic. Oshii is a good suggestion, but I can't really imagine who could do it. *ponders*