Fwiw your suggestion is something I've been trying to convince maintainers. One step at a time ;-)

None of the following is too radical or strange - opengl/es (thanks Laurent) and opencl - have been doing it for a while.

Individual steps prefix shows current status:
- Missing: Create a virtual driver package, one for each API: vaapi, vdpau
- Missing: For each API/frontend, add the respective optdepend of the virtual foo_driver
- Almost: Update API/frontend users to _not_ (opt)depend on the driver, but the API/frontend library
- Almost: No drivers depend() on the frontend. They all makedepend() on it.
- Optional: add a meta package for intel and amd akin to the nvidia one. It pulls all the respective vendor vaapi, vdpau, opengl/es, opencl drivers.
- Unrelated: libva-vdpau-driver and libvdpau-va-gl are unmaintained but working drivers. There is no reason to remove them for the time being

Added libva-mesa-drivers and removed libva-vdpau-driver to and from libva optdeps locally, intel-media-driver is already included. Will be shipped with the next libva release.

Is libva-mesa-drivers an error (the current package is called libva-mesa-driver - no "s") or is there a new virtual package not yet available on the repos?

That's an error, fixing it now.

Please check mesa 22.3.1-2 in testing, i've added libva-driver and vdpau-driver 'provides' in their respective packages.

Thanks for the update Laurent. For the sake of clarity, here is an update of the outstanding (sub)tasks.
If you agree, making it a proper TODO under https://archlinux.org/todo/ is an option.

Happy to provide help - provide patches or otherwise.

vaapi, drivers - libva-vdpau-driver, libva-intel-driver, intel-media-driver, libva-mesa-driver
- all drivers but mesa - demote libva to makedepends, provides=(libva-driver)
Side note: the intel-media-driver is special on a number of levels, it seems to link against libva.so so we'll need to keep the "depends=libva" for the time being.
- libva - remove all drivers from optdepends, use optdepends=(libva-driver) instead
- vlc, kodi, ovenmediaengine, obs-studio, others(?) - remove all drivers from make/"hard"/opt depends, use depends=libva instead

vdpau, drivers - nvidia-utils, libvdpau-va-gl, mesa-vdpau
- all drivers but mesa - demote libvdpau to makedepends, provides=(vdpau-driver)
- libvdpau - add optdepends=(vdpau-driver)

Humble poke? As mentioned before I'm happy to help in any shape and form.

Fedora folks say libva-vdpau-driver is old and broken. Ideally, libva-nvidia-driver would migrate from AUR to Extra first...

https://mastransky.wordpress.com/2023/01/31/firefox-va-api-and-nvidia-on-fedora-37/
https://fedoraproject.org/wiki/Firefox_Hardware_acceleration
https://aur.archlinux.org/packages/libva-nvidia-driver

Sounds like we're conflating multiple unrelated things. I think each one should be its own bug:
- the unofficial nvidia driver from elFarto should probably be picked to community - vote on the AUR?
- aside: should probably finish my upstream MR for ^^
- libva-vdpau-driver being old/whatever - open separate bug?
- proper dependency handling - this bug

This is an automated comment as this bug is open for more then 2 years. Please reply if you still experience this bug otherwise this issue will be closed after 1 month.

This should be mostly implemented now. Is anything missing?

Having a closer look a lot is fixed although some bits remain. There are four types of glitches:
- [1] optdepend on some drivers, instead of the virtual driver
- [2] makedepend on some drivers, instead of the virtual driver or frontend alone
- [3] drivers do not provide the virtual driver
- [4] drivers hard-depend on frontend when they should not


frontend libva, virtual driver libva-driver
- 1 - libva, obs-studio, vlc, ovenmediaengine
- 2 - kodi
- 3 - libva-intel-driver, intel-media-driver, libva-vdpau-driver
- 4 - libva-intel-driver

frontend lib32-libva, virtual driver lib32-libva-driver
- 1 - lib32-libva
- 3 - lib32-libva-intel-driver, lib32-libva-vdpau-driver
- 4 - lib32-libva-intel-driver, lib32-libva-vdpau-driver

frontend libvdpau, virtual driver vdpau-driver
- 1 - libvdpau
- 2 - kodi
- 3 - libvdpau-va-gl, nvidia-utils
- 4 - libvdpau-va-gl (should be testdeps only)

frontend lib32-libvdpau, virtual driver lib32-vdpau-driver
- 1 - lib32-libvdpau
- 3 - lib32-nvidia-utils