We just started a Discord server that focuses on MiSTer development. You can find it here: https://discord.gg/qkFvphP
A handful of us wanted to try formalizing the process a bit more, so I've been experimenting with getting continuous integration going, and setting up a branching model that's more similar to gitflow workflow. To test that, we've made a little Github org where we can work together. You can find that here: https://github.com/OpenVGS
I have CircleCI doing builds against quartus-lite 18.1 to validate that Pull Requests build before they can be merged. I'm also starting work to run testbenches if they're supplied in the core, as a way to do "unit tests". I'm hoping this helps to stop certain kinds of regressions, but we'll see. We understand that at a certain level, integration testing will likely be manual, but automating what we can should hopefully still be valuable.
We hope to be able to show the value of our experiments and get those changes back into MiSTer-devel, if people are open to it. Regardless, code changes to cores and Main_MiSTer will definitely be headed back upstream to the main project.
Thanks so much to Sorgelig for such an awesome project to work on! I really hope that our experiments are useful to the main project!