It would either look like 'Death Note', 'Monster' and 'Psycho pass' (a lot of background shading, characters lseem serious but keep that manga look to an extent) or be somewhat like 'Naruto' if the series is supposed to be longer. Obviously, I like the MADHOUSE style of animation, so I'd go with that. Mamoru Hosoda and MADHOUSE style fusion for an animated film.