This has always been a thought of mine. I wanted to replace/remove any non-free kernels with a libre kernel that doesn’t provide non-free binaries. I know some people say it does not matter, but I think it would be a cool idea.
I was thinking about hardening QubesOS by possibly using a deblobbed version of the Kicksecure kernel on all VMs. It would also come with Javascript disabled entirely on all VMs, all non-free binaries removed, everything run over Tor by default, maybe creating a custom repo with only free software, etc.
This would be inclined towards people who use Libreboot/Canoeboot, specifically those that use systems like OptiPlex 9020 and T440p, that run only 100% free software in the BIOS.
I’ve never attempted it, but I would like to know if anyone would be interested.
Further to this: Debian libre already exists and is exactly this. Probably the best way to start. That gives you Libre Qubes v0.0.1 nearly for free.
Then, for hardening: since Kicksecure is also just a hardened flavour of Debian, you could reduce your work to integrating the hardened config and kernel build of Kicksecure into Debian Libre.
Finally: it is worth considering whether you want to be attempting to deblob Xen - if that is necessary. It may make the project unviable as a one-man side-project.