Is there any chance that Duo will create a more advanced French course? I am doing Memrise, also but prefer the Duo format.
Occasionally they update the trees and add more content - that is it.
My two cents: try another site, for example, http://apprendre.tv5monde.com/fr/niveaux/b1-intermediaire. or https://www.fluentin3months.com/french-reading/. You can choose the level etc.
There are no advanced courses in any language on Duolingo, and I've not seen anything to suggest that they're planning any. I agree that they'd be great for those people who have finished their tree, myself included in French, but I doubt that there's sufficient user demand for them, especially when there are so many languages that haven't been touched at all.