Well, they could have an interesting exchange of magical knowledge. Shirou, with his reality marble skills, might be intrigued by Merlin's ancient and powerful magic. Merlin, on the other hand, could see potential in Shirou and perhaps offer some guidance on how to further develop his abilities.
One possibility is that they team up for a quest. Shirou's determination and combat skills combined with Merlin's magic could make them a formidable duo. They might face powerful foes together, and during their journey, Shirou could learn more about the nature of magic from Merlin, who has centuries of knowledge.
In the 'shirou meets zelretch fanfiction', Shirou might learn some unique magic from Zelretch. Since Zelretch is a powerful mage, he could have a great influence on Shirou's growth as a mage.
In this fanfiction, Shirou might have an epic battle with the dragon. He could use his skills and determination to face the powerful beast.
A plot twist could be that Merlin is not who he seems. Maybe he has an ulterior motive for meeting Shirou, like using him as a pawn in a larger magical game. This would completely change the dynamic between them.
Well, Shirou might start by being in awe of the nature spirit. Maybe the spirit shows him some hidden parts of the natural world, like a secret glade filled with glowing flowers. They could form a bond where Shirou protects the spirit's territory from those who would harm it.
They might have a tense encounter. Given their past history in the original story, Merlin could be cautious. Maybe Morgana has changed in the future, but Merlin still has his doubts. He might be on guard, ready to use his magic if needed.
Perhaps Zelretch decides to show Shirou different timelines or alternate realities. This could be a way to both amuse himself and also give Shirou a new perspective. It could also lead to Shirou having to make some tough decisions about his own path based on what he sees in these other timelines.
One possibility is that there would be a great deal of tension. Old Merlin has seen so much and has his own wisdom, while Morgana has her own powers and motives. Maybe Merlin would try to persuade Morgana to abandon her dark path. He could use his experience and the bond they once had to appeal to her better nature.
Well, in such a fanfiction, there could be a lot of self - discovery. Maybe the old Merlin gives advice to his younger self. For example, he could tell the younger Merlin about future challenges and how to avoid some big mistakes.
Well, it could be about many things. It might be a story where Shirou has an encounter with a character named Blue. Maybe it's an adventure they go on together, or a situation where they meet by chance and their lives change in some way. Without reading it, it's hard to say exactly.
At first, their relationship might be one of master - servant formality. Shirou would be cautious and respectful, while Merlin would be somewhat aloof. But as time goes on, Shirou's determination and pure heart could win Merlin over. Merlin might start to see Shirou as more than just a master, but as a friend or someone worthy of his real help.