I have a few suggestions for cars to add. None of them are particularly new, however, they are in the game files already.
First, I think the Ford Roadster should be added to the Offroad Builds. The closest thing I have to a Mad Max car is my Offroad Trans Am Superbuild. Then maybe add some extra customisations, exclusive to the Offroad Build of this Car, like maybe some bars, spikes, new exhausts, the option to remove windows, maybe even let us decal the engine.
We can turn this...

...into this!

Or this!

Wouldn't that be awesome?! And, there's lots of deserts about in Fortune Valley, so there's more than enough space to reenact Mad Max!
Which leads me to my second topic.
We have the Ford Crown Victoria Police Car. We have the Range Rover Sport SVR.
Both of those were in the game files.
So, why don't we have this beautiful beast machine?!

We need the Mac Attack! We have the two vehicles I mentioned earlier in the game, and in order to drive them before, you had to use mods. Now they are both driveable vehicles in Free Roam. So where is Mac's treasured Big Rig? If a mod is installed, we find out that the Mac Attack even has customisations available for it!
We can do so much with it! We have a Chevrolet Camaro in NFS Payback, so we can customise it to look like Optimus Prime and recreate Transformers.

We can use the folded-down car-carrier option as a Ramp for epic jumps, and awesome pictures.
We can use the Mac Attack in single-player races, and make hilarious videos about it, which would convince more people to buy the game, just for the truck.
We can even combine it with the Offroad Ford Roadster to recreate that scene from Mad Max Fury Road!
'The Scene': https://youtu.be/W28kNzz50pw
I don't care if EA make this truck something you have to buy on the market for £10. I'd still buy it! If they capped the Maximum Level for this truck below 399 (like they did with the Ford Crown Victoria at LV 365), I wouldn't mind at all!
I'm hoping that there are people out there who will see this post and agree with it. If they don't, that's fine. But if they do, they need to spread the message to EA. "Add the Mac Attack to NFS Payback Free Roam as a driveable car!"
Who knows? Maybe if they get enough requests, EA will finally allow players to own their own Mac Attack. They did it when everyone wanted the 299 LV Cap removed. So, maybe we can do it again.
Enough ranting, my final idea (which is a huge stretch and highly unlikely) is to give cars already in the game Superbuilds, for certain classes. We can even make them look like other movie cars.
For example,
1969 Dodge Charger Superbuilds:
Offroad

Race

1965 Ford Mustang Superbuilds
Drift (Improved)

Etcetera...
Please let me know what you guys think.