More story ideas I don't think I'll ever end up doing.
Let's say Arthur let's out his emotions by writing letters, and he's done that with everything for his whole life. Today, Merlin found the more recent ones. They were love letters... To him.
'It all started when Merlin found the love letters.'
Merlin goes crazy from years of helping Arthur with no credit. So now he no longer stays at Gaius' and goes in a hoodie at night. Every single night he kills anyone that tries to kill Arthur. Resulting in large casualties around Arthur. Often without an explanation.
After being stabbed, Person says "it swords of looks like you've got a problem with me,"
Gwaine dared Merlin to make a scene on a hunting trip, and start shouting at Arthur for leaving him alone that night. Arthur catches on that this is a prank, and gets close to Merlin, he then says: "so sorry Merls, maybe I can make it up to you..." He then gets close, puts his arms around Merlin's waist, and licks Merlin's neck. Then Arthur leaves promising more, also leaving behind a very embarrassed Merlin, and a nearly crying from laughing so hard Gwaine.
What if Hunith wasn't a nice mother? What if she was abusive. Then Arthur finds out, after she died. (So he doesn't go and kill someone in Cendred's kingdom and start a war.)
Merlin hasn't spoken a word in years. Write what made him go mute, and also write what causes him to speak again.
What if Uther had a habit of shipping people together, then playing match maker with them. What if, thats why Uther put Merlin as his son's manservant instead of any other job.
Write about someone (Merlin) unpacking a suitcase.
Past few nights, you've been in my dreams doing weird _____ and now I can't look at you properly
At a certain age, Merlin gains the ability to shape shift into a Dragon. At first he has trouble with it.
Arthur found the blueprints of the castle. This wouldn't have been a big deal if it weren't for the fact that there was a room in the blueprints that didn't exist. Write what he finds when he looks for it.
Merlin saves Uthur's Life, and now Uther knows Merlin has magic. What does Uther do, and how does Arthur react?
For some reason every single criminal, bandit, and possible enemy is terrified of Merlin and runs when they see him. Arthur begins to notice this. The enemy doesn't even care if there's a whole army behind Arthur, but gosh almighty the scrawny little twig steps out and the enemy full out Sprints away screaming "WE'RE IN TROUBLE FUDGING RUN YOU IDIOTS!!!"
Start a chapter with, 'it was on fire, and it wasn't my fault this time.'
Arthur wakes up a human lie detector. He realizes Merlin doesn't tell the truth about everything. On one mission, Arthur decided to try and figure out what his manservant has been lying about.
Arthur is turned into a Dragon, and instead of trying to find a cure, he's just like heck ya! I'm a flipping Dragon! Morgana can I please pretend kidnap you, and keep you in a tower for a while? Oh and Merlin clean my scales. (Arthur is still a Prince at this point.)
Arthur is Going to save a rumoured princess that is stuck in a tower. Little does he know, the damsel is actually the villain.
Write about a day where everything goes right for Merlin, and he's freaking out underneath truth to figure out what's going to go wrong today.
Story prompt: "you've looked better, what happened?" "I ran into some old friends"
Arthur and Merlin gain the ability to communicate through their minds. Explain how it happened, and what they use it for. Or Arthur gains the ability to mind read, and show what he hears from people.
(Modern) There's a magic club at school, and everybody is always learning new magic tricks to show the group. One day, a new kid comes in and he's good at magic. Like really good. Then there's the fact that weird stuff keeps happening around school.
Arthur rises out of the lake, but Merlin doesn't remember him.
When Merlin finds out that one of his uncle's died and left him a ton of money, strange artifacts, and a new place, what will he choose to do.
Have Camelot in grave danger of some sort.
"There is only one who can help us young Pendragon. Emrys."
"Where will we find this magical Emrys?"
"He is said to reside in Camelot."
Morgana takes away all of Merlin's memories of Camelot. At some point he lets slip that he has magic.