I would love this option as well. Duolingo has the "incubator," but even that is a slow and tedious process since the Duolingo system is the same for everybody (effectively). It has to be vetted before standard users can use it or else they'd (rightfully) complain.
For example, I'd love to give Lojban a go (constructed language), but because it is so obscure, I'm limited to basically a few Memrise sets and the CLL. I haven't used Lingvist for very long, but it is so effortless and has already helped my French out. If Lingvist had an option to (manually) enable community courses, and a basic SDK to design one, I'm sure language users from all over the globe, no matter how obscure, would love to participate in sharing their skills.
Of course this could be a logistical nightmare for the Devs, and possibly could scare off new users if not hidden well enough (there could be incorrect/incomplete courses which would discredit Lingvist for some users). On the other hand, it would really create a unique learning opportunity for hundreds, if not thousands, of languages that the Devs themselves would never be able to manage themselves!