We’ve been considering allowing 3rd party runtimes to be submitted to Flathub, so that it’s possible for different OS and distro vendors to participate in the Flathub ecosystem and give app developers their own choice in tools, support/lifecycle, updates, etc.
This is partly in response to Elementary OS switching their SDK to a native Flatpak runtime (per Distributing runtimes and sdk) and including it on Flathub would make a very smooth process for Elementary apps, and partly recognizing that folks like RedHat and Fedora are now publishing their own Flatpak runtimes which people are interested in using too.
I appreciate this is a trade-off; the more runtimes we have, the more bandwidth, disk space and memory we’re using on people’s systems, so our recommendation is still that unless you have a specific need to change tracks (eg your app is already developed and tested against an existing platform and you want to maintain that environment in Flatpak too), you stick to Freedesktop SDK derived runtimes wherever possible.
However ultimately Flathub is trying to reduce friction for app developers, so this move would allow them the choice of what works best for them and their users after evaluating the above caveats. We also want Flathub to remain neutral regarding OS/distro vendors, so there’s no benefit to us being strongly partisan on which runtime/SDK people use if it serves to locks platform providers out of our ecosystem in ways which could be unhelpful in the longer term.
That said, we need to make sure Flathub continues to provide a good experience for our users and we mitigate some of those adverse affects, so I’ve tried to draft a definition of what we’d like to see in a Runtime on the Wiki in order for Flathub to be willing to host it. I’d love any feedback and if folks have any suggestions about what a reasonable submission flow would be.