Cool; that makes sense! The addition of more entities, like artists, is also part of what we intend to do with AURA.
A big item on the to-do list for the AURA project is to implement a new beets plugin—alongside the existing web plugin—that provides an AURA implementation. If this kind of API is what you’re after, perhaps I can convince you to implement (and perhaps improve along the way!) AURA instead of inventing a new protocol from scratch? This could be a fun project, and thinking carefully about how the API should work might make it useful for other developers as well.