OKAY I'VE PROCRASTINATED THIS FOR LONG ENOUGH. HOW TO TRAIN YOUR DRAGON HEADCANONS. NOW:
Valka is bisexual and Gobber is gay, they became friends before Valka met Stoick (of course she knew him, cause he was the chief's son, but they hadn't become friends/lovers yet).
Valka and Gobber had been in the same gay friend group since they were teens.
Gobber actually introduced Stoick to Valka after her girlfriend broke up with her. She was really heartbroken and swearing she would never date a woman again; in a way she was right.
Gobber, at some point between becoming Stoick's best friend and presenting him to Valka, had a crush on him and asked him out; Stoick was surprised and flattered by this, but politely declined. After that their friendship was completely unaffected by this.
Hiccup is trans, no doubt, and when he came out of the closet he chose his name based on the story of his Great Grand Uncle Hiccup Horrendous Haddock II, "the dragon whisperer". He was a dragon simpathiser that Hiccup admired and read about in his family's sagas; although the sagas don't say anything good about him and his story is quite tragic, Hiccup III felt some sort of connection that he didn't understand (he realised soon enough) and so he chose his name accordingly.
His coming out thing is lowkey hinted at (no, of course it's not canon, let me dream) during the movies and the series: he continously wants to be like other vikings, he talks about how he disappointed his father because he wanted a more vikingish son, he compares himself with other vikings, when he meets his mother she tells him how he takes after her (even physically, if you think about it), etc.
Valka instantly respects her son's new name and pronouns because she is also queer, and if you're thinking "there's no way she knew his new name, she wasn't there when he chose it" I'm just gonna remind you this particular viking won't shut up, so he surely told him "I'm Hiccup, son of Stoick the vast" or something like that while she was taking him to the nest and she brushed it off cause she knew it couldn't be possible, then she saw the scar and her queer little brain made everything fit together.
Hiccup gets both crushing gender dysphoria from seeing himself reflected in her mother and an incredible amount of gender euphoria as he realises his identity doesn't need to be that performatively masculine and that his mother is quite queer herself, so in the end being like her is something he prides himself on.
In the third movie we see flashbacks of Hiccup being a little boy and he is called by his chosen name already, so we know he wasn't closeted for a long time and also that he came out really young.
At some point in "Race to the edge" Tuffnut calls Hiccup "sister", this hints to the fact that Tuffnut is also trans, and they sometimes call each other "sister" jokingly. Before he came out of the closet his name was some other consonant followed by "uffnut", but he swapped it for the "t" (for trans).
The sentences "Excuse me, barmaid. I'm afraid you brought me the wrong offspring. I ordered an extra large boy with beefy arms. Extra guts and glory on the side. This here. This is a talking fish bone." are so trans of him, he even uses "offspring" instead of son. This is also why Stoick feels really bad after telling him "You're not my son", realising just after he closes the door (which doesn't even shut completely, like the closet's) that he has disrespected and disregarded his son's identity. He "fixes" this later in the movie when he tells him how proud he is (p r i d e).
Astrid is also trans, t4t, love wins!
In the third movie, Valka and Gobber have accepted Eret in their gay best friend squad; they even fight together as a team at the end. But there's more, because Gobber spends the movie flirting with Eret and it shows they are quite close. Furthermore, when the dragons leave, Eret and Gobber stand together, and then, during the wedding Gobber is holding the bouquet and when it flies out of his "grasp" Eret is the one who grabs it. They are gay, your honour, I rest my case.
Eret is sooo gay, he does not only like men, he actively dislikes women romantically and sexually. We can see this in the way he refuses to even pay attention to Ruffnut's attempts at seduction. In the third movie he has already told her that he is gay, and she has taken it badly in the "I'm going to take this personally, not as something that just is" cause she is a bit dumb (but we love her).
Last headcanon for today is the main characters' pronouns and sexualities: Stoick (straight (up) go/ne), Valka (She/they, demisexual bisexual), Gobber (He/him, gay ass), Eret (He/him, closeted gay until the last movie), Hiccup (He/him->He/they after the second movie, bisexual ace), Astrid (She/they, bisexual), Fishlegs (He/him, bisexual), Snotlout (He/him, deep-in-the-closet gay), Tuffnut (He/him, aroace), and Ruffnut (She/her, straight).