As much as any software is ever "finished", I'd say TTG is finished. That is, I think the structure is ready for us to consider e.g. Introspective Template Haskell (
https://gitlab.haskell.org/ghc/ghc/wikis/template-haskell/introspective), which may be what you were thinking of when you wrote the sentence above. This would be a good deal of work, but I think it would move us forward nicely, and I think it's a reasonable time to contemplate doing this, if one were motivated.