Honestly? I think people sometimes miss the nuance in the Sonata/Aria dynamic because they get too hung up on the music metaphor. Sure, 'aria' as a solo piece and 'sonata' as a structured form is a starting point, but the real character growth happens in the fics that treat those terms as emotional archetypes. I read this one crossover AU where Sonata was a rigid, by-the-books detective and Aria was this chaotic, performative vigilante; the whole arc was Aria forcing Sonata to improvise and break protocol to solve a case, while Sonata gave Aria a framework that made her chaos actually effective. It wasn't about one changing the other, but about them building a third, better method together. That feels like the core to me: growth through synthesis, not replacement.
I've also seen it used to show regression, which is fascinating. A fic where a character who's achieved an 'aria' level of self-assured solo power has to relearn the basics of collaborative 'sonata' structure after a trauma—that's brutal, but it rings true. The terms become a language for tracking a character's relationship to themselves and others, more than just a simple 'they become stronger' progression.