no one is going to buy into cosmic turtle bullshit that King loves so much.
Attach Joss Whedon or JJ Abrahams to the project and they will call it a masterpiece
Good luck selling audiences on a movie where the central character fucks a Shoggoth loli girl who feeds on semen in order to remake the world in her image while also having the titular character turn the nothing female supporting characters into fodder and a mindless fuck slave. Because somewhere between 'central character fucks a Lovecraft' and 'feeds on semen' is where the studio starts demanding a rewrite until you get Jennifer Lawrence's boring ass shitting up the Song of Sara.
I know you are only being about 80% serious with this but I'll bite any way.
Like I said, the Loli shit has no bearing on the central theme of the story whatsoever. You could age Saya up to say 19 and get the'creepy innocent' effect that was achieved in Ex-Machina.
You seem to forget there were
3 supporting female characters in Saya No Uta. The Doctor was essentially the anti-hero adversary. Hollywood continues to get away with far far far worse in regards to female representation so I can't see this being a problem.
Women being eaten by monsters is about as old as cinema itself so I don't even know why you're pretending that is an issue.
Being turned into a fuckslave would be controversial but could be fairly tame if the shift it more to a 'timid servant' and play up the fact that she had a crush/ was already attracted to the main character so the sex isn't non-con just being turned into a Cthulhu is the main source of anguish. Again, not huge take away from the story. She is still a 'former friend turned into a monster' which is what the plot is asking for.
'Feeds on Semen' is a unnecessary eroge detail to amp up the blowjob scenes. The analogy is clear, she is a black widow spider/brood mother/plague. They could easily change it to 'his sexual energy' or 'his life force' and nothing is really going to be lost in adaptation in terms of plot.
This isn't anywhere near the first movie where sexual themes are explored with a non-human entity. It was pulled off with Under The Skin, Teeth, Splice, and it's predecessor 'Species' and pretty much any 'weird science' type movie with a higher than 12 rating. These movies get made.
It's funny you mention Jennifer Lawrence because while I think she is an overrated peice of breathing furniture. She actually starred in Mother! Which is probably more disturbing than Saya for most people and it still got a bunch of awards and made it's budget back.
Once you remove the unnecessary shit Saya isn't as hard to adapt as your trying to make it out to be.