Do the discussions have to be related to Duolingo?
Or can they be, for example, Do you watch/like Doctor Who? Or, who likes dogs?
I'd say the discussions have to be about Duolingo or about a specific language taught here because there are sections for this: Duolingo, Spanish, French and so on.
But it would be nice to have a forum with topic about anything we want to discuss. There could be sections for discussing anything in a specific language, that would be fun and useful.
With the current forum design I wouldn't recommend opening discussions for casual topics as those might drown discussions about the website. It would be nice to have a place where you can chat casually in the language that you are learning, although it could potentially be distracting.