I'm woefully ignorant of the movie industry, so I wouldn't know where to start with producers or directors. I don't even really know any actors aside from the ones who made blockbusters in the 90s. -_- But I'll give a go at what I can.
Used Books is really long, so it's more serial, even a miniseries might work but it would need a completely stand-alone screenplay different from the webcomic form.
Tbh, I have often pictured Sandra Bullock as Kaida and Aaron Eckhart as Fudo but I never thought about other casting. The other characters are mostly younger, and I'm not terribly familiar with younger actors. All my actors who'd make choice villains have sadly passed away irl since I began writing.
If it went into the long-running drama format, it might be a good fit for Netflix and released in seasons. Maybe one of the premium movie channels.
I guess PG-13 (or PG, since PG-13 is a pretty 90s era rating). TV-14 maybe. We could amp up the language and nudity to appeal to the drama audience and kick up the rating.
Not really big budget. It is mostly set in one modern city (could probably film in any city since it's made up – or we could set it in a real city for the production). CG is not needed. And not particularly destructive. (It's not like blowing up a car every scene or anything.)
Alternatively: It might make a good high-quality pseudo-anime series.
Summary is tougher. I'm not sure I can put it into words, atm. And it would depend some on what the screenplay becomes and what aspects it really focuses on. It could be portrayed more lighthearted or it could be melodramatic. In the current market, the latter might sell better.
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This is the poster I made for
Strange Creatures for the DD Award promotion. It's also basically my cover page because it has some deeper meaning.

I might actually present the story in sections, so it could be a two-part or trilogy. I think it could be in theaters.
I'd want a cast of unknowns. Although if a big name wanted on-board and screen tested well, that'd be okay.
To be honest, I'd kinda like to see what Henson Studios could do with it. Maybe not as the main production company but a partnership. Alternately, I guess CG would be useful. Or a combination. Disney might actually do okay with it considering some of their live action properties. (It should be live action. I cannot picture it animated in any form.)
Rated PG. Really nothing particularly offensive. We could cut the tobacco use out or other things if it's a hot button issue.
I don't know how much stuff costs, but budget might be a little high. It needs some effects and multiple locations.
I guess the promotion would probably start as horror/monster movie feeling but cut into the (human) characters being sarcastic at each other. I think that would set the tone well.