Someone else already took The Rescuers so I'll say Robin Hood. I don't know why they thought Tarzan would work. The Princess and the Frog. I don't think Hercules could work in a serious way, only tongue-in-cheek or as children's theatre.
I know Pocahontas has never been the most popular or financially or critically successful but I wonder if there's a way to scale that down and make it work. Probably not since a great deal of what sells the movie is the visuals and you just can't recreate that epic scope. Also, with the whole serious/darker vibe they'd probably try to add John Rolfe and that's a can of worms you don't want to open.