I love how freaky the low men are. That story's the one from Hearts in Atlantis isn't it? That whole book is heartbreaking and beautiful. Think he nailed their otherworldliness, like the lost pet posters, and I love the idea of them driving grotesque 50s style Cadillacs and so on.
I would be really interested in reading/watching some sort of pre-prequel following the last years of the great old ones, showing us how they merged tech and magic and destroyed themselves 10000 years or whatever before Roland's time. In my head I see the aesthetic as being a mix of like Silicon Valley tech, steam punk, magic realism, Blade Runner, Final Fantasy 7 & 8. Could even have Flagg working with Maerlyn to being the world to ruin, etc. Whole new cast of characters, explore how they built the cyborg guardians, Lud, Blaine, etc. Think I should pitch this to Marvel.