- Shadows of Ecstasy: some critical moments (e.g. Roger Ingram trying to make sense of what he has seen in the inner room) are interior experiences that need a little verbalization. A few scenes need really good acting plus a little special effects goosing. The whole "dark continent" component wouldn't go over well; we know the rest of the world too well. I'm iffy about this one; the premise is hard to visualize and I didn't find the premise compelling.
- Many Dimensions: A few minor special effects for the Stone's properties, but some critical moments in Choe's transformation are interior experiences. I think this could work, but the ending might play as a huge downer--the plot might need tweaking.
- War in Heaven: With a few special tricks here and there for magical effects, and some explanation of what the Grail was, I think this one could make a good movie. You have to get an excellent John, though.
- The Place of the Lion: A few special effects, but you'd have to motivate the release of the Forms much more strongly, and be explicit about what Forms meant. ("It's all in Plato, all in Plato: bless me, what do they teach them in these schools") I think it could be done--it might turn out better than the book (not my favorite).
- The Greater Trumps: It needs lots of special effects. A few critical moments are interior (you get different characters' points of view)--voice overs for Sybil? This could easily be a nice dramatic movie with plenty of action.
- All Hallows Eve: Straightforward special effects for wizardry and the city of the dead, but there are a lot of different characters' points of view--lots of extra dialog to be added. It would be worth trying. Unfortunately a completely unrelated movie exists by the same name.
- Descent into Hell: It has internal transformations that I think would be hard to portray, especially Wentworth's deepening rejections and the "bearing of burdens". I think it could be done, but I'd be nervous.
For some of these you'd probably only have Williams fans buying tickets, and the purists would gripe. I think The Greater Trumps would be the most likely to appeal to a broader market. I wonder how much you'd have to trim to make a screenplay of it.
If you are in the USA, where the copyright rules were adjusted by a Disney-rented legislature, you're not supposed to click the above book links.