Marking the victory :)
I don't know, maybe this idea had already been expressed, but there are too many discussions to look through, so: wouldn't it be nice to have some kind of a sign (a star or another symbol) that you get on your profile picture, showing that you mastered a course (together with the beautiful victorious Duo statue you get additional symbol on your profile pic). Wouldn't that be nice and informative :) ?
Maybe a little version of the owl for every language you have mastered. And als a flag of your native language.
And some discussions should be deleted when the problem is solved.
Maybe they could just frame the flag beside your username golden or make it a little bigger or something.