An open GPU design would be great for the RaspberryPi for instance, even if performance wouldn't be competitive with NVIDIA or AMD (it really doesn't need to be). I think a "RISC-V, but for GPUs" would make a lot of sense, e.g. RISC-V met a similar scepticism in the beginning, yet it seems to quickly gain steam in the last few years.
Executing shaders better than Nvidia and AMD is not likely.
Selling good graphics adapters at competitive prices to concrete users is even less likely.
Experiments with APIs would have a fatal adoption problem.
Avoiding DRM, if legally feasible, would be less useful than spending the same resources to support Sci-Hub or improve laws.
And of course for the more practical purpose of writing mere software, including Vulkan implementations, specifications are complete and open enough.