Yeah, I agree. I thought Miller blew a great opportunity to create a new character -- played by Tom Hardy -- set in the original Mad Max universe.
In my imagined screenplay, Hardy would have been someone looking for the original Mad Max. In fact, there were rumours for awhile that he was playing the Feral Kid grown to adulthood.
Eventually, he would have either discovered:
i. the remains or grave of the original Mad Max; or
ii. an older, even more cynical, Max who lives a hermit existence, but sacrifices himself to save Hardy and/or another band of survivors, thus exiting the series.
That would have left Miller free to continue the saga as its own stand alone franchise. I originally pitched this idea on the old IMDB forum a few years before Creed essentially did the same thing with the Rocky franchise. And the idea of a new character searching and finding a character from the original movie(s) was used in Bladerunner 2049. I think Hollywood scriptwriters stole my concepts! Any lawyers out there?
Anyway, as it stands now, this one movie is likely to be an outlier. In my mind, it's NOT a Mad Max movie, just another post-apocalyptic action flick.
However, I also heard the rumour that Gibson was offered the role, but turned it down because he considered himself too old. Which makes all of this just a fun bit of speculation. Ironically, during a recent Graham Norton interview, he mentioned that that he has had informal discussions with Danny Glover and Richard Donner about doing another Lethal Weapon.