The love potion could make Arthur see Merlin in a whole new light. He might suddenly be more trusting of Merlin's magic, which is usually a big issue between them. For Merlin, it could be a chance to get Arthur to listen to his ideas more easily.
In a fanfic like this, Merlin might accidentally give Arthur a love potion. Arthur could then fall in love with the first person he sees, which could be hilarious or cause chaos. For example, if he falls for a random servant, it would disrupt the normal order of the court. And Merlin would have to find a way to reverse the potion's effects without getting into too much trouble himself.
It could be due to jealousy. If Merlin gains too much power or popularity in Camelot, Arthur might feel threatened and in a fit of rage, stab him.
Merlin would likely be seen as a bit of an enigma at Hogwarts. His magic is different from the norm, so he might be a bit of an outsider at first. But his kind heart and loyalty would soon win over his classmates. Arthur would be the brave and headstrong character, perhaps a bit like a Gryffindor version of his usual self. He'd be eager to learn about combat magic and would probably be a natural leader in group projects or Quidditch teams.
In a Hannigram fanfic, a love potion could work by heightening their existing emotions. Since they already have a deep, if somewhat twisted, connection, the love potion might intensify their feelings of attraction and obsession. For instance, it could make Hannibal more protective of Will or make Will more accepting of Hannibal's darker tendencies.
The love potion can initially create a false sense of love. For example, if Draco is under the influence of a love potion for Harry, he might act overly affectionate, which could make Harry uncomfortable at first. But it could also be a catalyst for them to start interacting more, which might lead to a real relationship later.
It could also complicate their relationship. If they find out it was a love potion that made them feel this way, they might question whether their feelings are real or just the result of the potion. Naruto, being a straightforward person, might struggle with this idea. He wants to believe that his feelings for Hinata are genuine, but the existence of the potion makes him doubt.
In some fanfics, Merlin might have tricked Arthur or convinced him it was for the kingdom's safety.
Obviously Merlin himself could be affected. He's the one with the magic and might accidentally expose himself to the potion. Arthur is also a likely candidate. As the king, if he's affected by the love potion, it would have a huge impact on Camelot. Morgana could also be a character affected. If she were under the influence of a love potion, it could change her plans and her relationships with other characters. And Guinevere too, since she has important relationships in Camelot that could be altered by the potion.
It would become much more complex. They might start off being confused and a bit awkward around each other. But then, as they start to explore their feelings, they could become closer in a more intimate way. Their friendship would be transformed into something deeper, with a new level of understanding and trust.
If Merlin and Morgana are siblings, it could create a lot of tension. Morgana's actions against Arthur might be more complicated. Merlin, being loyal to Arthur, would be torn between his sister and his king. He might try to convince Morgana to support Arthur, which could change the power dynamics in Camelot.