For starters, drop the idea of a single rigid taxonomy — there are too many ways through, even assuming that canonical representations are possible. So we’re probably looking at something at least personal, possibly community-based. Folksonomic tagging would be a start, but how to navigate in a neatly glanceable fashion?
I’m thinking of building a personalised acoustic surface, where patches of looped sound are snippets representing genres and subgenres, and which morph into one another ‘at the edges’ so you end up with a navigable 2- or 3-space which is a musical patchwork. ‘Drill down’ into any patch and explore the subgenres under it. Everyone (or their software) could generate their own personalised surface — which would keep things small enough to be navigable — from a CDDB-style online database of user-contributed, tagged patches. Easy to make, with a few rules and some code to allow individual people and their software to stitch them together seamlessly into their own personalised surface. And would appeal to long-tail passion: people who care about genres would be motivated to have them well-represented by nice patches…