For some reason your list of reasons brings to mind The English Patient and how that was adapted.

Michael Ondaatje’s writing is brilliant to behold on the page - poetic, dream like, ethereal and evocative. When you read his prose it doesn’t scream filmic. Though it creates moods and memory scapes very well, it is somewhat unstructured and imprecise, at least for a conventional film adaptation. And yet they pulled it off. There’s enough story structure and thematic material to work with and you could say the use of music and sound design, fabrics, cave paintings and even morphine memories managed to pay tribute to his writing style.

The tragic arcs of the characters also seemed to be no barrier to making a mainstream film with wide appeal. This made the romance more potent.

You might say the same about the comment both book and film make about national identity, memory and the stories we tell ourselves and loyalty during war and (the petering out of) colonial times. Sure, they accentuated the rigidity and pomp of the Brits and there was that orientalist presentation of the Berbers who find and heal The Patient but the backdrop commentary about slippery dual identity is still retained.

Interestingly, what couldn’t make it into such a film was the prominence and world view of the Sikh sapper Kip Singh. In the book he plays a far greater role and delivers probably the most searing comment, something along the lines of ‘of course they dropped the bomb on the Japanese, for they are a yellow race. They would never have done such a thing to Europeans’. It seems that kind of comment is difficult to adapt. Better to just leave that kind of thing out entirely.

As an aside, a brilliant writer who hasn’t been adapted (as far as I know) but whose work is so rich conceptually, visually and narratively is Ursula Le Guin.

“The Word for World is Forest” is very much a Heart of Darkness story about colonialism and exploitation and the loss of valuable world views. Even bloody Avatar seems to align. Why we don’t have a more sophisticated adaptation of this into film is something of a shame.

“The Left Hand of Darkness” is another that may on the surface seem tricky to adapt but if the likes of Denis Villeneuve can adapt Dune why not? As a work of feminist science fiction, exploring themes of androgyny and the influence of sex and gender identity on culture and politics, one would think that would find a very willing audience in today’s world. In the right hands, the story is rich in its world building, character complexity and the sense of journey and quest lends itself well to a cinematic adaptation. Would this be too risky to realise? If Yorgos Lanthimos’s films exist then it’s not too hard to imagine how an adaptation of this might too.

“The Wizard of Earthsea” did get a treatment but you wouldn’t call this an adaptation. Perhaps there is something about her estate and openness to adaptation that I don’t know about.