The story that wasn't completed
Merida is responsible for Mildew's death.
Hiccup is blackmailed into marrying an insane Duke.
The Duke would be a teenager. I based him off of a mixture of Kronos and Death the Kid from Soul Eater. On the outside he's neurotic, cool and stylish. Tell him no and he's a panicy, unhinged oddball with psychotic tendencies. He wants to get his way and is not use to hearing no. It happened once and since then the Duke's been terrified since of it happening again. The Duke is also lonely and wants friends. He uses his high standing to make people do what he wants and because it's a natural he doesn't find it cruel or controlling.
He meets Merida, Hiccup really, and grows to like 'her'. The Merida he heard about was fierce and brave. He meets a pretty red-head with heart and wit and grows to like her more.
Hiccup-stuck-in Merida's body says no to an engagement. After that a monster no one can find attacks people in the DunBroch castle.
The Duke tells 'Merida' if she agrees to 'spend time with him', he'll protect her kingdom from the monster.
p.s. Hiccup-as-Merida is less shrill to Merida's friends. They are amazed with this the boys do new stuff. MacGuffin wants to visit art museums, Dingwall wants to shop for new clothes and Macintosh wants to sing in a bar. The Duke stocks them after a weird date with Merida. The Merida he hung out with talked to animals and candle-sticks, brushed her hair with a fork and said she heard music when no one else did.
Back on Berk, the New Hiccup is adored by most. Merida-in-Hiccup's body loves the more hard-handed way Vikings live.
Tuffnut is a dim, gross, destructive dude- everything her mother would hate. He knows how to have fun and Merida grows to like him, more than his sister.
The imperfect things are mainly naggings from Astrid (who likes new Hiccup's spunk but misses his concentration on dragons) and a mopey Toothless.
What she can't stand is a mean Mildew (wrote this before the season finale).
Merida doesn't take his attitude in stride. She and the twins pull a prank.
Stoik chews New Hiccup out for not being the better man. Mildew gives a snide remark.
Merida as Hiccup yells at him something along the lines of- "You are the worst person I've ever met- and I wish you'd drop dead! But instead of that, I hope you live- I hope you live in the miserable, lonely, hateful life you've created for yourself forever."
Then Mildew drops dead- to spite Hiccup.
The funeral is awkward because no one is sad. In a guilty, weak state Merida makes out with Tuffnut. This disturbs the people who find them because they see Hiccup making out with an indifferent Tuffnut.
The wedding reception night is crashed by the teens of Berk.
Why would they go all the way there? Astrid and Stoik would rather believe in magic spells than Hiccup getting it on with Tuffnut. And Merida remembered the magic pendant had a deadline before becoming permanent.
The boys dress as servants and the girls 'borrow' dresses from party guests.
All together they figure out the monster is a dragon.
Change-wing I'd prefer. They find him in the Duke's ship. The condition they find the dragon in is appalling. He was trained cruelly and it's lead him to be insanely vicious and hostile.
Imagine the Duke on the crazy dragon vs Merida and Hiccup riding Toothless.
The end of the Duke's dragon goes like this.
The Change-wing's wings are shot down against a few trees. Its feral and stuck where he is and will never learn happiness. It's a pitiful sight.
Hiccup sees no hope in the animal. He slices the neck, while still in Merida's body.
Furious at the Duke, Merida and Toothless bring him to a dungeon.
Toothless corners him, and Merida says, "Your evil will never spread."
The Duke leaves the castle, alive, but with bloody pants.
The kids are lucky because there's a single day left before the curse is over.
So the last night of the curse, Hiccup and Merida go under the moonlight and reconnect the necklaces that broke.
There's glowing, magical stuff and badda bing- Loki appears!
The Vikings kneel while the Scotsmen stare at the god in disbelief. Tuffnut and Ruffnut are ecstatic because their huge Loki fans.
Loki admits he caused the switch for some fun. The God says he chose them because Merida is a princess who is destined to bring shame her family legacy. Hiccup is the boy who gave dragons to Vikings; which will cause far more destruction to the world than help to his own people.
Loki got his laugh so the two are switched back.
Merida wakes up in Hiccup's clothes. Hiccup wakes up in a dress.
Oh, and Merida introduces Tuffnut to her family.
Ruffnut-"Twins run in our family."
Fergus-"Triplets run in ours."
Before they all split, everyone goes back to the bar to sing "Rock you like a Hurricane".
Adopt it if you want.