As to the car bombs, I could give or take them. But I would like to se new art for the catapult. Perhaps the catapult could be of metal on the frame of a car or truck?
I would also like to see a catapult or trebuchet on a working flatbed truck.
Well the problem I see with catapult and/or trebuchets is what I've said before, other than siege engines, they are pretty much useless. The unit density is such that you don't have masses of tightly packed infantry that could potentially be threatened by such weapons. With low density populations, you'd want to maximize your combat power by using Cavalry (early mobile infantry) and vehicles since it is very difficult for armies without them to affect those with them.
So then you get back into attacking cities. As I've pointed out, the barricades of Fury Road are nowhere near the elegance or science of some Vauben fortress, but would be mostly stacked up rubble pushed there to clear the area. Attacking such with catapults isn't going to do a lot. Chucking rocks into the city itself will probably destroy the very stuff you're wanting to take.
So from a cost to effectiveness ratio, catapults and trebuchets are pretty much useless. Now big dart throwers like ballista would be more useful since they are more of a direct fire weapon that could be used to attack hard-points along the 'walls' but even then, it's only a siege weapon; you're just not going to be able to kill that many troops on the ground with it.
So, instead of wasting time, resources, and effort on a weapon that is practically useless, better to go about up-armoring things like tractors, dozers and other large vehicles. I mean Caterpillar built the first tanks for the US Army. As mentioned earlier, these tank-dozers would be better at attacking walls and could also haul "battle-trailers" like modern day elephant howdahs, sort of like how they up-armored the oil trailer in "The Road Warrior" On a stable platform like that, you could easily mount an air compressor to run those dart guns you see in that movie, which in my opinion is probably one of the best, low tech weapon you could build prior to large scale manufacturing of early model machine guns like the Maxim gun or the Vickers water-cooled gun.
Plus, even with the fall of technology, making canons isn't that hard in the sense that the technology isn't that hard. Hell, outside Vancouver barracks there are two canons made by the Fort Vancouver High School metal shop. I mean technology is hard to eradicate. All it takes is a few books kept by one survivalist, and you're going to have the know-how to build things like canons. The problem is getting the tools to make the tools to make the item in question. So early canons wouldn't be that hard to make as long as you have access to a foundry capable of making bells and that sort of technology has been around for hundreds of years. I would say that by the time any Civ is up to building enough gunpowder to field old-style canons, they would have plenty of people who knew the basics of how to make them since so many people would have had all the metallurgic experience due to all the scavaging and reworking of things.