I assume, judging by the title being Fury Road, that the entire movie might take place on the road. That'd be pretty great. Sort of a gigantic gas powered caravan, which would make the movie basically a two hour chase scene with a little bit of story at the beginning and the end.
The other idea was that the road might be sort of a place where people in cars sort of fight to the death... Like Thunderdome, but with cars.
Basically, my only worry is nothing to worry about. I just want it to be nothing like the other three. Road Warrior was entirely different from Max, and Thunderdome was way different from the other two. If you watch Max and then Thunderdome without seeing MM2 in between, you'd be surprised that they were part of the same series.
Another thing is the continuity of Max's body. Every injury leaves scars that we can see in later installments. Example, his knee cap in MM1, leads to the leg brace in Max 2 (I like when the mechanic oils it for him).
I've got an idea that MM4 will have a storyline similar to the type of thing we saw in THE MASK OF ZORRO and BATMAN BEYOND, with an ageing Max sort of "training" a younger character to be his replacement. It could begin with Max meeting up with a boy in his late teens-early twenties who has narrowly escaped a gang of really vicious desert barbarians who slaughtered his family, and who now aches for vengeance. Max could then take him under his wing.
It turns out that the barbarians who killed Max Junior's family are searching for hidden nuclear and/or biological weapons that are hidden somewhere on the other side of a vast road in the wasteland. Over the many years following the destruction of civilization, the road and its secrets have become the stuff of post-apocalyptic legend, and the barbarians talk about it as "The Last Highway of the Old Folk, laid out in the time before the Age of Chaos" or something like that. Max and his protege have to fight them in order to avenge Max Junior's family and to stop them destroying the world all over again! Tell me what you think, eh?