You could make Hiccup's banishment a turning point for character growth. Have him face various challenges and learn important lessons. Maybe he returns with a new perspective and skills that surprise everyone. Another idea is to introduce a mysterious mentor during his banishment who guides him.
One way could be to have him go on a solo adventure to prove his worth and find a way back.
Well, you might have other characters secretly helping him or have him encounter new allies on his journey. Maybe he discovers some hidden power or knowledge that could change his fate. Also, you could throw in some unexpected twists and turns to keep the story exciting.