La Valse d'Amelie - Amelie (Original + Reharmonization + Countermelody) - Yann Tiersen

This sheet music starts as an accurate transcription of the original theme for accordion and then evolves into an original rearrangement with new chords and bassline and after that a new countermelody.
The piece starts beginner friendly and ends being very difficult since you have to manage two voices on the keyboard while playing many chords and a bassline on the bellows, but I guarantee it's very fun and rewarding to play once you learn it. I think it's a very good piece to measure how much you've grown as an accordionist to try to play at different stages of your development.