Really Good Suggestion!
(idk if anyone has already said this, but if someone has, I am sorry)
I think it would be cool if Duolingo offered extra lessons that you could buy for lingots that include different slang terms or words that only exist in certain countries. For example, I am going to be moving right next to Mexico soon, and it would be nice if the Duolingo Spanish course offered me a lesson on common Mexican slang and words that are specific to Mexico. This would be especially useful for languages that are used throughout the world and have therefore developed differently.
They do have three lessons you can buy for lingots: Flirting, Idioms, & Christmas (only available to "buy" around Christmas time)
I agree with you that it would be nice to have additional lessons. In fact, this seems like something that volunteers might be able to produce if Duolingo allowed it. It'd be great to have something to spend lingots on -- especially something helpful to learning the language as opposed to a silly suit for the owl.