A Furiosa movie is like if they made a spin off about Tina Turner of Bartertown. These are interesting people with interesting worlds, but they functioned to hinge Max's story on. And going away from Max kind of betrays that point.
I get in a sense these were influenced by westerns and samurai films, where the stranger falls into a place with a specific problem and gets involved. Making a spin off about the people there misses the point and is a step back. They weren't about the place. The place and people are over with when the hero rides out. No matter how popular they got in Furiosa's case. Would it have been cool to see a Lee Van Cleef Angel Eyes spin off with Leone directing? Maybe, it might sound really cool like Furiosa, but it misses the point.
It being a prequel doesn't really help. This is Miller and I'm sure he could make something interesting out of it, outside of that point, it just seems creatively limiting. He could do anything and chooses a spin off?
Unless Miller does something totally different with it where he can get experimental and create something of a wholly unique identity and Furioisa is a jumping off point. Otherwise I have zero interest in seeing a prequel story involving Imogen Joe or the concubines. Been there done that. I'd rather see a wholly different Miller film like the one we're getting or another Max film that expands the world.