Perhaps a plot where Shirou is accidentally transported to the Fate Apocrypha world before the start of the war. He then uses his knowledge of the Fate universe to try and prevent the war from happening. Along the way, he meets the various characters and tries to convince them to find a more peaceful solution. This would involve a lot of character interactions and moral debates.
An interesting plot could be that Shirou's reality - marble somehow merges with the Fate Apocrypha world. This causes strange phenomena and he has to team up with the local mages and Servants to fix it. During this process, he discovers new aspects of his own powers and also learns about the deeper secrets of the Apocrypha world. There could be a lot of exploration of both his character and the world - building in this scenario.
Chapter 348, "Schemes and Counter-Schemes, the Five of East Ridge, Zhu Yang's Threat, The Dust Settles (Part 5)", is the clearest early answer because it turns Zi Yang's situation into a concrete plot event. The chapter turns that into something concrete by moving from Zhu Baixu, challenge ring number 1." "Lin Qiang, challenge ring... to Leader, something's off, Wang Wu is heading towards ring number 1!"....
Chapter 375, "Jin Siyue and Dogs Not Allowed Entry", is the clearest early answer because it turns Mr's situation into a concrete plot event. The chapter turns that into something concrete by moving from He Pingzhang spoke eloquently, but the moment he opened his eyes,...
Chapter 51, "The Great Lodging Competition", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from Oh, okay." After uttering two syllables indifferently, Jiang Li bent...
Chapter 157, ""Forced onto Liangshan", is the clearest early answer because it turns Director Sun's situation into a concrete plot event. The chapter gives readers a real answer by opening on It was already the last day of Flash Change in his dream.
Chapter 60, "The Human Oil King", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.
Chapter 410, "There's Something Wrong with This Lottery Box!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from Nina's voice had barely fallen when Pan Xing turned his gaze to Jiang...
Chapter 486, "So It's All Set Up, Right? Then She Will Set Up Too!", is the clearest early answer because it turns Yu Li's situation into a concrete plot event. The chapter gives readers a real answer by opening on Hearing Jiang Li's words, the audience in the live stream was also...
Chapter 137, "Lai Fu's Death Has Another Cause", is the clearest early answer because the violence around Zhao Lang becomes final instead of hypothetical. Early on, Jiang Li personally went to Lai Fu's tomb and performed a small....
Chapter 363, "A Cure-All for Fancy Trappings and Arrogant Tyranny", is the clearest early answer because the rescue effort creates visible on-page results. The chapter turns that into something concrete by moving from I bid 550,000!" "600,000!" "750,000!" "....." Soon, the price of the...
Chapter 221, "No Choice, He Gave Way Too Much", is the clearest early answer because it turns Director Sun's situation into a concrete plot event.