It’s easy to treat this as “books I really like and wish could be made into movies” so that’s what I’ll do. 🙂
I’d like to see one of Michael Marshal Smith’s novels made into a movie. However Spares got the closest to being made, then along came The Island, whose central premise was the same as one of Spares’ ideas, and which wasn’t very good, so the chance of Spares being made vanished. That said, Only Forward is still my favourite and while it’d need a lot of CGI, it may be possible. Jeamland weirdness might be hard to render effectively.
Rats and Gargoyles by Mary Gentle is such a chunky book that condensing it into a movie would be hard, even before you got into practical issues such as recreating a continent-sized pseudo-Renaissance city or how to realistically portray those central characters who are humanoid rats. Ash probably beats it in the unfilmability stakes though, as the structure where the main plot is framed by correspondences by a translator/author who is finding some increasingly bizarre events being described in supposedly historic documents would be hard to translate to the screen.
Oh, and as far as Watchmen goes – the plot of the comics transitioned to the silver screen pretty well, the look wasn’t too bad (a bit shiny) but Jack Snyder completely missed the tone, in my opinion.
It probably wouldn’t be as hard as Ash, though,