Using mpv-libplacebo as the playback backbone of Kodi would be a gamechanger imho
Kodi on Linux is a big fancy (and deeply integrated) wrapper around libav/ffmpeg so any idea using mpv is a non-starter. It's also not required since libplacebo can be integrated with ffmpeg, as mpv (which is based on libav/ffmpeg) does.
However, libplacebo is intended for x86_64 architecture devices using OpenGL/Vulkan, which doesn't align with ARM SoC boards that use OpenGLES; althrough really it's about the formats used and where to implement the shaders required. The zero-copy pipeline we use with GBM also poses a challenge as it assumes you take a raw decoded frame and move it through the pipeline to the output stages without copying (or transforming it in software, which inherently requires copying). Hence ARM SoC's handle transformation in hardware during decoding (while extracting the raw frame) or in hardware using a image post-processor in the output stages. As mentioned before, RPi5 is lacking those hardware functions.
NB: Tiben20's current experiments are targetting Windows.