Okay, but imagine a BBC Merlin time travel AU where both Merlin and Arthur get sent back, but neither one of them knows that the other one got sent back so they're both trying to change things while also not arousing suspicions. And, in true BBC Merlin fashion, they don't find out for the longest time. And I think they would get sent back at, like, the beginning of season two, that way Merlin and Arthur's dynamic is already established, meaning both of them don't want to do anything to change that and arouse the other's suspicions.
And you would think that one of them would notice, but every time Merlin does something suspicious/different, Arthur just thinks, "Oh, he's probably doing a magic thing that I didn't catch last time." And whenever Arthur does something different/suspicious, Merlin's just like, "Ah, my actions are having a butterfly effect. Good."
You've got two people trying to keep Morgana from turning evil. Arthur is telling her that things will be different when he's King, while Merlin is trying to relate to her more. Arthur automatically knows who to trust and who not to trust, and when Merlin points those things out, he actually listens. Arthur is always in the know. Uther has no idea what's going on. Merlin is out here doing ALL THE THINGS with his magic, and Arthur doesn't even notice. And neither of them can tell when the other is lying because both of them are such idiots. Somehow, Arthur ends up in mortal danger just as often as he did last time.
But it's not just silly-goofy-fun-and-games. There is so much angst. The whole time, Merlin is missing his Arthur, the one who died, and he thinks that he's never going to have that Arthur again. Plus, he knows this time that, if he fails, Arthur will die, and so there's a whole other level of pressure there.
Arthur, on the other hand, doesn't want to put Merlin through all of the trauma that he faced before. He feels responsible for making Merlin feel unable to share his secrets with him, and also knows that he will be causing Merlin unimaginable grief if he fails, which he does not want to do. Again.
But the whole thing is mostly hijinks. Because the show is mostly hijinks.