MLP AUs for BBC Merlin always fall flat for me because I'm never satisfied with how people reconcile the ban on magic with the innate magic that every pony possesses. Because MLP has specific rules for its magic system and it doesn't really fit with the magic system of BBC Merlin. I've thought about it a lot myself but I can't come up with a satisfying compromise.
So, MLP AU but it's just that The Elements of Harmony exist in Camelot and the knights turn someone into stone using the Power of Friendship (which, for those that don't know, is a tangible force that turns people into stone, amongst other things).
They go on a journey to find this supposed weapon to defeat the Camelot equivalent to Discord (Draconiqus, god of chaos, might just be the same Discord all things considered. He just took a vacay) while Arthur has been made captive and arrive to a forest full of trials where each of them goes through a trial for one of the elements , without knowing they're being tested. They then reach a clearing with a tree whose fruits are gems (I know that's not how the Tree of Harmony works in MLP but I figured it would be cool to go with the magic of the Earth bit for this) and when they pick them, they turn into jewels (because if they turned into swords it would be too easy. Now they've gotta use magic to save Arthur. Also: Merlin in a tiara).
Helping someone falling off a cliff and telling them to let go and that it would be ok because they can trust you (the ground was closer than it looked)
Helping an injured animal when everyone else was too scared of its appearance to look past it
Laughing in the face of fear to get rid of it
Giving up something precious to you to help someone else
Saying no to the opportunity of a life time when that opportunity involves abandoning your friends (who are also in danger at the moment)
For Magic there isn't really a trial. It's Twilight's personal trial of realizing the importance of friendship. The moment she does —once the other elements are together— the element of magic reveals itself. I'd imagine something similar could happen with Merlin too. I think the trails fit the knights I chose pretty well.
They elements (and associated pillars) would be assigned like this:
Honesty – Strength (Lancelot)
Kindness – Healing (Percival)
Generosity – Beauty (Elyan)
Arthur gets to be the princess
I know Gwaine is strength and Arthur courage in BBC Merlin, but I like to think that's a different sorting system for a different quest. Because this one is about friendships, not fights. Also otherwise Leon gets left out and I don't like that. Merlin is just universally considered magic.
Gwaine complains about going from strength to laughter.
But they don't know what each element stands for —except for the magic, but that's a given it's Merlin. He's going to have to channel the other elements through his and it's going to be a mess. Definitely a magic reveal— so they're going to have to find the right book that explains more than what the one that told them about their existence did.