Oh, gosh, no, that sounds like a fantastic AU idea! I know for a fact that other fanfic writers have tried their hand at having Bog try becoming a Fairy for Marianne’s sake, whether he only does it temporarily or with the intention of it being permanent differs.
Maybe he goes to Plum - or another Fairy like her, since he still has some issues with her over the Love Potion - and asks for a Potion that would let him switch species for a day. To show Marianne that if it would be easier for her, easier for her Kingdom and her Father, he would make that change for her.
And all the Fairy girls are throwing themselves at this tall gangly nerd of a Fairy who just happens to be a fucking babe, and Bog has no idea what to do with all this attention, he thought the suitors Mother brought by were bad, this is even worse.
And another Fairy after the Crown tries to pass himself off as Fairy!Bog, only for Marianne to see right through him and head-butt him and tell Bog that he’s a freaking idiot. “Bog…I want what any princess wants. To live happily ever after… with the Goblin I fell in love with.”
BUT OMG, WAIT, WHAT IF ONE OF THE EFFECTS OF THE POTION WAS THAT IN ORDER FOR IT TO WORK, THERE NEEDS TO BE AN EXCHANGE OF SPECIES?! BOG, A GOBLIN, BECOMES A FAIRY, A FAIRY NEEDS TO BECOME A GOBLIN.
BUT BOG DOESN’T KNOW THAT AND HE TAKES THE POTION AND HE BECOMES A FAIRY.
BUT THEN MARIANNE WAKES UP TO FIND THAT SHE’S BECOME A GOBLIN.
HIJINKS ENSUE. EACH OF THEM EXPERIENCING WHAT THE OTHER GOES THROUGH.
AND DAWN AND SUNNY ACCIDENTALLY TAKE THE POTION TOO AND THEY BECOME AN ELF AND A FAIRY RESPECTIVELY.
HOLY SHIT, I WANNA DRAW THIS NOW!!!