I believe there is a way to accomplish this without seeking input from people on Reddit or message boards for new domains to contribute to.

There are lists on Github that curate libraries native to a particular programming language. For example, there is a list for Lua (https://github.com/LewisJEllis/awesome-lua) and another for Python (https://github.com/vinta/awesome-python). Explore these lists to identify areas that may require assistance. Some of these lists have not been updated for years, so it is worthwhile to conduct additional research on the domain before undertaking a project.

I have personally completed a project using this approach, although I did have some background knowledge in that domain.