The US probably won't do it (although comic adaptations are big hits) unless other manga adaptations were successful first.
Dragonball is well know all over the world and has a big fanbase, but the way they have been doing it, it'll probably not be a hit.
And it'll be some time until Alita is made. So there won't be more adaptations coming from the US (first one has to pave the way).
I don't know why the japanase don't do adaptations though. Mangas and animes are part of their culture so it should be logical to try real life movies for many of the series, but they don't do it. For whatever reasons.
They could at least give us some animated adaptations for some series in the quality of the final fantasy children advent (I'd actually prefer that). Great quality but still very similar to the manga/anime origin and a much better way to do it than in real life (for most series).
Dragonball animated that way for example would've been much better than a real life film.
The characters (body, hair and face) can look like they do in the manga. A human can never have such a hair, face or body so there is no point in trying it.
Plus you get japanese sound if it's done by them.
But they don't seem to be interested in real-life or animated adaptations.
Maybe one day. Gantz won't be at the beginning of the list though.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"
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