"I'm already happily engaged to Hiccup."
"Well, how are you going to have heirs?"
"My village chooses the heir based on skill, not bloodline."
Hiccup runs away
Gods and Magic is real.
Humans have the ability to connect to magic, but dragons instinctively use it
Hiccup lives and travels with Toothless learning how to use his magic after an incident "reveals" it to him
He eventually flies and meets another tribe of dragon riders. Who have lived with Dragons for generations learning magic from and with them.
They respect Dragons and can magically bond with them as partners
They use every part of fallen dragons. Bones for weapons and tools, scales and teeth for clothes etc.
They are in awe of Hiccup. Impressed by his knowledge and his bond with Toothless. None of them ahve seen a successful bond with a nightfury.
He stays with them for a while and to be fully accepted into their clan he has to complete their rituals of adulthood. including marriage.
Tattoos mark important milestones including bonding with a dragon.
Their courting rituals have to do with flying Compatability and hiccup does something amazing and the only one worthy of him is this badass dude (OC).
This will be back and forth between "present" which will be years after the movie divergence and "past" which will be hiccup's journey to this tribe
When he arrives back to Berk he's a confident man in charge here to deal with the dragon problem along with his other dragon warriors.
He stands up for himself and the clan follow him he and OC will be co clan leaders eventually and are taking the lead as practice
Oc gets hit on by a lot of village girls but he's like I'm engaged or maybe married by that point. Eventually their like with who and he's like Hiccup. They're like heirs? And they say they are already training a girl who shows promise and plan on taking her in as their own heir.