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 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 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.
One interesting plotline could be Shirou using his gamer skills to quickly analyze and defeat opponents in the Holy Grail War. For example, he might see the battle scenarios as levels in a game and use strategies he learned from gaming to outwit his enemies. Another could be him leveling up his own abilities in a way similar to how a gamer levels up a character, perhaps unlocking hidden powers within himself that were previously unknown.
The contrast. Shirou Emiya is originally a complex character, and making him an overpowered gamer creates an interesting contrast to his usual self. It's like seeing a different side of him.
One plot idea could be Shirou Emiya finding himself in the world of Fairy Tail and having to adapt to the magic - based society. He might try to use his Reality Marble in unique ways to combat the dark guilds. Another could be him teaming up with Natsu and his friends to take on a powerful, otherworldly threat that has somehow entered the Fairy Tail universe. He could also learn new forms of magic from the wizards in Fairy Tail, like learning how to incorporate Fairy Tail - style elemental magic into his existing abilities.
One idea could be that Archer is transported to an alternate universe where Shirou has a completely different personality. Maybe Shirou is a villain in this world, and Archer has to figure out how to deal with him. Another plot could involve them teaming up to fight a powerful, new enemy that neither could defeat alone. For example, a powerful mage from a different dimension who wants to conquer their world.
Another plot could involve Shirou reincarnated in a modern superhero world. He might struggle to fit in at first, but then realizes his abilities can be used to fight super - villains. His noble phantasm could be adapted in a unique way to combat the new threats in this world.
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.
One interesting plot idea could be that Artoria Archer and Shirou Emiya are forced to team up in a strange magical tournament. They start off distrusting each other, but as they face various powerful opponents, they gradually learn to work together and understand each other's motives better. For example, they might encounter a foe who can manipulate time, and they have to rely on Artoria's combat experience as an Archer and Shirou's determination to find a way to defeat the enemy.