A third plot idea might involve a young Shirou accidentally summoning Saber. Since he is a child, he doesn't fully understand the implications. Saber then has to protect him from other magical threats while also trying to figure out how to return to her own realm. During this time, Shirou shows his natural inclination towards heroism which Saber encourages.
Another interesting plot could be set in a scenario where Shirou, as a child, is sick. Saber, using her magical powers in a limited and gentle way, tries to heal him. This could lead to a deep bond forming between them as they share quiet moments during his recovery. Along the way, Shirou starts to idolize Saber even more.
One plot idea could be that child Shirou discovers Saber's true identity as a king through a magical encounter. Maybe they go on a small adventure in the local forest, with Saber teaching Shirou about honor and courage in a child - friendly way.
One idea could be Shirou trying to understand Saber Alter's dark past in Camelot. He might discover the events that led to her becoming Alter. Another could be them teaming up to face a new threat in Camelot that neither could handle alone. Maybe a powerful mage is trying to rewrite the history of Camelot and they have to stop it.
A possible plot is that Shirou as a girl gets caught up in a romantic sub - plot that was not possible before. For example, she might be the object of affection for a character who previously only saw her as a friend when she was male. Also, she could struggle with her identity, trying to hold on to the values she had as Shirou while adapting to her new physical form. This internal conflict could drive the story forward.
One interesting plot idea could be that when Shirou summons Merlin, Merlin initially seems uninterested in the Grail War but gradually gets involved to protect Shirou. He could use his magic to deceive the other Servants, creating illusions to mislead them about Shirou's true strength and intentions.
Another plot could be that Shirou discovers a new, highly competitive gamer community. He starts from the bottom but his determination and strategic thinking, which he has from his heroic deeds, help him rise through the ranks. He might make new friends and rivals in this community.
One idea could be Shirou finding himself in the Rosario Vampire world and having to adapt to its unique rules. Maybe he uses his skills from his own world to protect the school from various threats that are different from what he's used to.
Another plot could involve Shirou traveling to a future time where the world is in chaos due to misuse of the Grail's power. He has to find a way to reverse the damage and set things right. This would put him in a whole new set of challenges as he might face more powerful and corrupted enemies.
One plot idea could be Dark Shirou in a post - apocalyptic world where he uses his skills to survive and rule a small territory. Another could be his journey to find a way to reverse his dark transformation. Or a plot where he has to face his darker self in a psychological battle within his own mind.
One interesting fanfiction could be where Saber summons Shirou in a modern setting with a twist. For example, instead of the usual Holy Grail War context, it's in a world where magic is more commonplace. Saber might summon Shirou to help her with a different kind of quest, like saving a magical artifact from being misused. The story could explore their relationship development outside of the typical war scenario.
One plot idea could be Shirou Emiya, as an angel, arriving at the Highschool DxD world and getting involved in the ongoing power struggles. He might use his skills from the Fate series to help out the various factions while also trying to maintain his angelic identity.
Another plot could involve Shirou getting a glimpse into the past of the Grail War through some sort of magical artifact. He discovers secrets that were long buried and this knowledge could either help him in the war or put him in greater danger as others may want to silence him.