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.
In the future, perhaps technology has blended with magic. Merlin and Morgana meet in a world that's very different from what they knew. They could start by being hostile, but then realize they need each other. Morgana might have knowledge that Merlin lacks about this new world, and together they could work towards a common good, like saving the future from a dark force that's trying to erase magic.
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.
Their relationship could be more complex. Instead of the straightforward enmity of the past, they might be forced to cooperate due to new circumstances. So, it's no longer just about good vs. evil.
In the 'hogwarts meets the future fanfiction', there could be a great exchange of knowledge. The Hogwarts students could learn about future sciences like advanced astronomy or genetics, while the future people could be amazed by the magic of Hogwarts. There might be a character from Hogwarts who becomes a bridge between the two worlds, teaching future people about magic and in turn learning about their advanced technologies.
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.
Well, Itachi might be shocked to see how much Sasuke has changed in the future. Maybe Sasuke has developed new powers or a different outlook on life. Itachi could also feel a sense of pride or regret depending on how Sasuke has turned out.
In such a fanfiction, perhaps future Gohan could warn present - day Gohan about upcoming threats that he has faced in his own timeline. He might share his experiences and knowledge, which could make present Gohan a stronger fighter and a wiser person.
In this fanfiction, perhaps Stoick would be amazed by how much Hiccup has grown and changed in the future. He might see all the new inventions and ways of life that Hiccup has brought to Berk. Maybe there would be a moment of pride as he realizes his son has become a great leader.
In this fanfiction, Peter Pettigrew could find himself in a world where the wizarding society has changed drastically. For example, there could be new spells and a different social structure. He might struggle to adapt, as he is used to the old ways. He could also encounter descendants of his former acquaintances, which would lead to some really interesting interactions. They might judge him for his past actions or he might try to redeem himself in their eyes.
In this fanfiction, Sasuke might be shocked to see how much Sakura has changed in the future. Maybe she has become a more powerful ninja, or perhaps she has achieved great things in the medical field, which could make Sasuke view her in a whole new light.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.