One great 'powerful Merlin fanfiction' could be the ones where Merlin fully embraces his magic and uses it to reshape Camelot's future. For example, in some stories, he doesn't hide his magic from Arthur as early as in the original series and they rule Camelot together with magic being an open and respected part of the kingdom.
In this fanfiction, it could be that Harry's power stems from his experiences. He has faced numerous dark wizards and challenges, which have forced him to grow stronger. Merlin, on the other hand, might not have had such intense and varied experiences. So, through his trials, Harry has developed powers that surpass Merlin's.
Fanfiction.net also has a good number of Merlin fanfictions. Although it has a broader range of works, if you search specifically for 'powerful Merlin', you can come across some interesting stories. However, the quality may vary more compared to AO3.
Well, in some fanfictions, Harry Potter might be seen as more powerful than Merlin because Harry has faced and defeated Voldemort. That's a huge accomplishment. Merlin, while very powerful in his own right, didn't have to deal with a dark wizard as complex and powerful as Voldemort. Harry's ability to survive the Killing Curse as a baby also gives him a sort of 'chosen one' status that could be seen as making him more powerful.
Magic knowledge. Harry has a lot of knowledge from Hogwarts, which could be more comprehensive than what Merlin had. So in fanfiction, this knowledge could be used to show he's more powerful.
It's hard to say. Peter Pan has his unique abilities like flight and his connection to Neverland's magic which gives him an edge in his own territory. However, Merlin is a very powerful wizard with a vast knowledge of magic. If they were in Camelot, Merlin might have the upper hand due to his familiarity with the local magic and his experience dealing with different magical threats there.
For Harry, he might be shown with enhanced spell - casting speed and accuracy. His Patronus could be stronger or have new abilities. As for Merlin, his traditional powers may be toned down. Maybe his ability to foresee the future is not as accurate as it was originally depicted, making Harry seem more powerful in comparison.
In a more humorous take on the 'Merlin tied to a bed' fanfiction, it could be that Merlin has fallen ill and the overly - protective Gaius has tied him to the bed to make sure he rests. Merlin, being the active person he is, complains constantly. Arthur comes to visit and teases Merlin about finally being still for once. Meanwhile, Merlin tries to convince Arthur to let him loose, promising that he'll be fine. Eventually, Gaius relents and unties Merlin, who then immediately starts bustling around the room.
If Merlin died in the fanfiction, it could trigger a series of tragic events. The balance of power could tip, and the villains might seize the opportunity to attack. It could also bring out hidden strengths in the remaining characters as they try to cope with the loss.