The Avengers where building a time machine and Tony made a mistake and everyone in the room (Tony, Steve, Bucky, Natasha, Peter, Bruce, Thor, Loki, and other Avenger members) accidentally went back in time when dragons and Vikings roamed

meanwhile in your timeline

you and your friends, (Hiccup, Astrid, Fishlegs, The twins, and Snotlout) were riding your dragons and having fun. You have a [encert dragon species] named [encert name].

you all were flying your dragons, joking and messing round

Hiccup: Wow, what an adventure!

Snotlout: That was awesome!

the twins are arguing

You and Astrid were rolling your eyes at the twins' argument than suddenly a portal opens in front of you but you managed to dodge it

Astrid: What is that?!

Fishlegs: I've never seen anything like it...

suddenly, the Avengers fell out of the portal and landed on the grass below you all

Tony: WHERE THE F-. gets cut off by Steve

Steve: TONY LANGUAGE!

Peter: looks up uhh guys...

Natasha: Not now Pete- Notices you guys above them on your dragons

You and the other riders (Hiccup, Astrid, Snotlout, The twins, and Fishlegs) shared a glance

Snotlout: Who are they?!

Fishlegs: it looks like they came out of that thing..

Tuffnut: AWESOME!

Ruffnut smacks the back of Tuffnut's head

your dragon looks like he/she wanted to attack the Avengers but you tapped their head gently, signaling not to attack.. Yet..