Basically just a random idea I have where Shirou ends up meeting Shazam.
Summary:
After the death of Kiritsugu, Shirou struggle to improve his Magecraft. He struggled by the reality of how weak he is despite he promised to be a hero of Justice in Kiritsugu stead.
At the same time, Mamaragan/ Shazam is searching for an heir for his power, but instead of Billy Watson, he stumbled on a boy who are so broken and empty, Shirou. In a way Shirou is the most suitable vessel, as he was too broken to walk the path of evil. Heck, it take an eternity to him become a cynical bastard like Archer despite being enslaved and killing people while he didn't want to.
The Wizard summoned Shirou, and grant SHAZAM power.
With his newfound power, Shirou begins to save people left and right, and maybe you can also give him knowledge in handling his Reality Marble.
As for the setting, you can do it in Fate World where Shazam is just ancient power that manages to survive all the way to the modern world. Maybe Clock Tower will hunt him down trying to keep Magecraft hidden.
Or you can just make it that Shirou and Kiritsugu live in the DC universe, comic/ Animated/ movie up to you. Shirou meet Batman and Superman, then join Justice League.