It would be awesome to cultivate a lesson surrounding French slang.
Example how to say "dude" or "sweet" in French so that students can learn how a true French person would speak.
It could be an auxiliary skill like flirting and idioms. Students could buy it with lingots acquired in learning.
Just a suggestion.
It can also be learned on this forum by asking questions.
We can say 'mec' for 'dude', in French, but it is not as simple as that. You must not try to find out how US or UK slang translates into French, but how French people speak naturally. Translation is already a hard task (being a professional translator, I know a little about that), but with slang, the result is very rarely natural. So you must start from French slang and try to understand (not translate) it.
I agree. I'm a French teacher and teach my kids a little French slang, but let them know not all translates directly. It would be interesting to see something like that come about.