An interesting idea, The hell of Mad Max with humanity descending into anarchy and infighting is a pretty realistic place for Shirou to start losing it and turning into EMIYA, hmm horribly cruel idea.
Rin and some of the saner Magnus are horrified by how civilisation is self destructing and decide to take drastic measures to fix the world. So they recreate the Holy Grail and the servant system but in true nasuverse fashion things go to hell.
Greedy Mages make a play for the grail then local warlords get wind of the prize and a crazy Shirou gets involved and doesn't trust anyone with the power of the grail.
I don't think Shirou should summon Arthuria her purity and good don't really mesh well with the crazy world of Mad Max and she would have a great relationship with Shirou. Hmm maybe Mordred, she's tied to Arthuria's legend so summoning her should be possible for Shirou and the interplay between the two of them would be interesting.
Servant battles atop speeding vehicle over rugged terrain. Shirou and Mordred snarking at each other. Maybe it ends with Shirou making the bargain with Alaya to stop a ramping servant or to save people from the grail going nuclear.