This is one of my less-than-fully-formulated ideas.
But so Merlin gets cursed. Probably by Morgana (always a reliable bet), sometime in series 4 probably after Arthur's king. Anyway, so Merlin is cursed and he's in excruciating pain at all times except when he's touching Arthur. There has to be physical contact between them, then all the pain goes away immediately. Don't ask me why, I didn't figure that part out yet.
Gaius does research and says the spell is meant to drive people mad with the pain of it or something, and that a man's mind is liable to collapse and his heart to give out from the strain after a certain length of time. He's searching for a cure or a countercurse but he hasn't found anything yet.
Meanwhile, Merlin and Arthur are sort of stuck together, trying not to forget and lose contact because whenever they do Merlin is overwhelmed with pain. When night comes, Merlin steels himself to suffer the night, but as soon as he drops Arthur's hand to leave Arthur snatches it back, looking horrified. Merlin says he can't exactly stay there, Arthur says of course he can. It's super awkward at first and they can't get comfortable trying to hold hands, so Arthur finally huffs and throws propriety and all boundaries out the window and spoons up behind Merlin so they can get some damn sleep. (note: I am for once not thinking Merthur.)
They manage to get through a day or two like that, but Merlin gets more and more agitated as time goes on without a cure. Eventually he wrenches himself away from Arthur and tries to leave. Arthur chases him down but Merlin doesn't get very far before he collapses anyway. Merlin insists that they can't go on like this, that he's ruining everything for Arthur. Arthur says he can't stand to see Merlin in so much pain. He's seen Merlin take injuries before, injuries that would have laid a knight out, and joked about them, so he can only imagine how torturous this curse must be to affect him so. Merlin points out that Arthur can't rule like this. He can't train his knights, he can't go to council meetings, he can hardly leave his room. He can't go about dragging Merlin around by the hand like a disobedient child. Arthur says he can if it keeps Merlin from harm, they'll make it work. Merlin tries to convince him that he's not worth it, that his kingdom must come first, Camelot comes first, always. Arthur reminds him that he could go mad, that he this curse could kill him. Merlin understands that. Arthur gets angry and says Merlin's not going to kill himself just so Arthur can go to a damn council meeting.
I don't really have an end point for this one. But forcing them into close proximity when emotions are running high is always worth it, especially when it makes Merlin self-destructive and Arthur over-protective. And Merlin wouldn't even have a chance to try to break the curse, not holding Arthur's hand, and not when without Arthur because they pain's so much he can hardly breathe much less think. Somehow things would have to be resolved.
