𝘞𝘦𝘭𝘭, 𝘮𝘺 𝘭𝘰𝘷𝘦 𝘪𝘴 𝘢𝘯 𝘢𝘯𝘪𝘮𝘢𝘭 𝘤𝘢𝘭𝘭, 𝘊𝘶𝘵𝘵𝘪𝘯𝘨 𝘵𝘩𝘳𝘰𝘶𝘨𝘩 𝘵𝘩𝘦 𝘥𝘢𝘳𝘬𝘯𝘦𝘴𝘴, 𝘣𝘰𝘶𝘯𝘤𝘪𝘯𝘨 𝘰𝘧𝘧 𝘵𝘩𝘦 𝘸𝘢𝘭𝘭𝘴. 𝘉𝘦𝘵𝘸𝘦𝘦𝘯 𝘵𝘦𝘦𝘵𝘩 𝘰𝘯 𝘢 𝘣𝘳𝘰𝘬𝘦𝘯 𝘫𝘢𝘸, 𝘍𝘰𝘭𝘭𝘰𝘸𝘪𝘯𝘨 𝘢 𝘣𝘭𝘰𝘰𝘥𝘵𝘳𝘢𝘪𝘭, 𝘧𝘳𝘰𝘵𝘩𝘪𝘯𝘨 𝘢𝘵 𝘵𝘩𝘦 𝘮𝘢𝘸.

AHADFHAHKDA thank you I love this question.

I have lots of ideas for this AU, and a lot of it is still kinda in the draft stage so I really only want to share the stuff that is more or less fact (in this AU). As far as Jack, the basics about him is that he is not a viking, or from Berk originally. He's from a neighboring island close to Berk, and his village was attacked by a group of "vikings" when he was a child. Stoick had heard of the attack, and being it was a territory close to Berk, took a group to investigate to try and determine who this group was and what they wanted. The concern was that this group was advancing on territory and would be attacking Berk next, but upon investigation, their motives were unclear, as they didn't seem to take anything of value, and it appeared as if they just wanted to level the village and kill the residents. (huehue mystery) Jack is one of the survivors they pull from the rubble, he's about 8 years old at this time. He develops a resentment for vikings because he confuses the two groups, and doesn't trust the people of Berk. Even as he grows up in Berk, he always holds some level of resentment towards vikings in general (because he believes that vikings had killed his family and destroyed his village), and deliberately does not call or consider himself a viking.

Part of this resentment is also fed by Berk's bad relationship with dragons at the time he lives there as a child.

Jack and Hiccup meet when Stoick brings Jack back to Berk, and they're both around 8-10 years old. Hiccup's first impression of Jack is that he's surely a wildling or something feral. At first, Jack is taken in by Astrid's family, and this results in him and Astrid actually forming a strong, sibling-like bond later on. However as Jack grows, he becomes more and more independent, and grows up just as this kid that sleeps in the Great Hall, is fed by various families and is just kind of like a stray, and doesn't stay in one place for long. This is entirely his own decision, as he felt staying with one family was too confining and he deliberately did not want to "become a viking".

Jack finds an order of druids, and decides to leave Berk and join them around the age of 13-14. This is set right before the events of the first movie. He visits Astrid when he can, and when he comes back, Hiccup has changed the village into accepting dragons, and their relationship starts to develop more. Also, Jack's view on vikings positively improves, because of Hiccup specifically.

As for Hiccup's differences, he really hasn't changed all that much. The events of the first and second movie still play out (aside from his relationship with Astrid), because its essential to his character development. His relationship with Jack started when they were around 15, and I want to say they get married towards the end of the second movie, or shortly after, so they're about 20-21. With Jack being a druid, the order he's part of requires him to be away for months at a time, mainly in the winter. This puts a strain on the relationship for both of them, and Hiccup is always kind of dealing with Jack being the free spirit he is, while Hiccup wants that freedom but is chief of Berk. On the flip side, Jack wants his freedom but also wants Hiccup. He struggles to find his place between both worlds, and still feels like an outsider a lot of the time in Berk, despite being married to the chief.

That's basically the setup for the AU, and like I said before I have tons of ideas for the current timeline, but that's still in progress XD Thank you for your questions! <3 Here's some extra WIP sketches for this AU too~ (NEHEHEHE ANGST)