Coverage for the sake of coverage is mostly meaningless unless it verifies the workings, but tests for the functionality that hasn’t been tested yet are very welcome.
In general you can just submit such test contributions directly via github, but if you’d like to make a study and perhaps compile a list of functionality that’s untested yet, then yes, making a dedicated to testing thread under https://forums.fast.ai/c/fastai-users/dev-projects and linking to it from Dev Projects would be a very useful contribution, @Benudek. There is one for Documentation improvements so that would be one for Testing Improvements or something like that.