RotBTD headcanons
I do not care if the fandom is dead, I’m posting it anyway:
Hiccup and Merida are cousins. Valka and Elinor are sisters, Valka eloped with Stoick the first time Stoick arrived on the mainland (since Berk and the surrounding islands are somewhere between Iceland and Scotland, but closer to Scotland). Elinor received word of her sister’s elopement and then never heard from her again, and somewhat held a grudge against her for not doing her duty to her people and being so willful. Elinor sees a lot of her sister in Merida, which is why she’s so strict with her.
Jack is the only one who is from a modern era, he at some point is given the ability to time travel (thank you Father Time!) and he meets Hiccup first, before the first movie. Due to Hiccup’s belief in many things people don’t (trolls, etc) he can easily see Jack (though he expects an old man). He and Jack butt heads a bit but they are very special to each other.
Punzie doesn’t meet the other three until later. Hiccup spends a few years searching for information on different types of dragons not native to the islands around Berk. Accompanied by Jack, usually (though he does need to return to our time to do his duties as a guardian), and Merida when she can get away (they both eventually have their own dragons, but I’d need to look up which ones again) her her and Hiccup bond quickly, and she’s sorta his best friend in the group. Jack gets along amazingly with Eugene. They double date about it.
Merida and Jack fight often, but it’s more of a sibling bickering type.