I'd rather a sequel to Fury Road that follows Furiosa and how she handles leading her new people as well as battles with the other broken factions who also have new leaders stepping up. It would also be interesting if Furiosa discovers that she's slowly turning into another Immortan Joe because she realizes what he did was out of necessity in order to ensure the survival of his people in this stark environment and she's making the same decisions despite hating her life under his regime.
A prequel seems unnecessary because I think we know all we need to know about her past. I don't think it's vital that we're shown her childhood, how she became Imperator, or how she lost her arm. Leaving the details to the imagination is better imo since we had glimpses of her past in Fury Road.