I was always a bit frustrated by the way nbconvert pulled in pandoc as a dependency, but then didn't make use of any of its power (pandoc surely has a powerful AST). Instead they re-implemented filters and templates themselves. It struck me that it would have been much cleaner to write a proper 'ipynb > AST' Pandoc reader, and then expose pandoc AST filters and templates.

I was a bit suspicious of moving away from pandoc to sphinx, because sphinx seems markedly less powerful and general than pandoc. However, you seem to think it's the opposite! Does Sphinx have an AST? I can't see any reference?