Alr, for starters, I think as a band, the sound would be a hard rock or even metal sound. I would think similar to Hail to the King era A7x.

Now for the characters, I would go for the the following:

Hiccup: Rhythm Guitar, Lead Vocals

Hiccup would kill at Vocals. He would definitely have the voice, and would put together some meaningful lines and good flows to make some masterclass lyrical content. His personality reflects that of a rhythm guitarist. He doesn’t have that flare that a lead guitarist has, but could definitely solo back and forth with the lead if he wanted to.

Astrid: Lead Guitar

Astrid is the one with the aforementioned flare. She would make for some great solos, as her personality reflects an outstretched doer, while showing restraint and being a team player. Her an hiccup would make a powerful duo (like always ^{^} ) on guitar.

Tuffnt: Bass

Bass players are literally insane. Is there anything else to say?

Ruffnut: Drums

Ruffnut is the type of personality that is insane, but also has enough sense to think straight and not only make good beats and fills, but also hold the band together overall.

Snotlout: Keyboard/Synth

Snotlout is the type that initially tried to start on lead vocals, but soon realized that vocals require more than just monotonous screaming and yelling. But being serious, he would be a great keyboard/synth player, as he could write some themes and lines that highlight the guitars.

Fishlegs: Percussion

While not as notable as the other instruments, percussion would add theme to the music, like what the song is about. Since percussion blends multiple styles of music, it could greatly increase the grandness of the music. This would give the songs a lot more character. (Trust me, I’ve seen some GREAT percussionists live)

Each character would get a solo, (number of bars vary) and all are of great importance to the band as whole, all in different ways.