Re: Batman III: The Untold Story That Takes Place After Returns
This sounds like such a cool idea. This really appeals to that confused 6 year old in me that was left hanging waiting for a true sequel to Batman Returns.
I also think you're making the right call to be a little more free about what you explore in the story. Sam Hamm and Daniel Waters delivered two very different screenplays with very different styles. This isn't like Nolan's universe where there's a specific style of writing for each film, so I think as long as you stay true to Burton's visual style,*it'll feel like a natural extension of that universe.
I'm also not sure what the legalities are since it's copyrighted characters and such, but I would totally buy a print of it when it's done if you made it available. It'd be a cool thing to put on the shelf.