actually yes you could design an alternative API and even implement it....

there's HW references + the linux drivers for AMD (both UMD and KMD), there's refs for NV and their open linux KMD (no UMD though), intel also has HW refs and open drivers for linux

I think it would be possible to put together as long as you don't mind only working for certain hardware, like a prototype you know

I might give it a shot to make a proof-of-concept "high level low level" graphics API ngl if no one else feels like so...

I think there's space for a "two-level" design like have the driver consist of the HW-specific parts, expose things in terms of GPU VAs, explicit sync, explicit memory management, command buffers etc. then have a high-level wrapper over it to provide vaguely GL-ish semantics, and you could intermix the two APIs, obviously you wouldn't be able to get the VA for a high-level object because that'd break the autotracking, if you use low-level you're responsible for syncing, cycling etc.