I'd prefer a sequel over a prequel to track the fallout of what happened, but that really wouldn't be a Mad Max film unless Max somehow ends up getting roped in with some other party that eventually interacts with Furiosa again (like perhaps she is corrupted by her new role and someone else takes her down).
That being said, I don't think any -quel with her would make sense for Mad Max. In the end it's basically a series about a wandering reluctant hero and how he eventually helps a group that he didn't want to be involved with. In a way Max is never truly the main character, he's just a passer-by that stories would be told of years later, a ghost and legend. The people he helps are the core of the story. Please, *please* make a 5th, but keep doing what the others did and take him somewhere new.