OneAPI is already cross platform through codeplay’s implementation which also can run on NVIDIA GPUs, its whole point is to be open cross platform framework that targets a wide range of hardware.
Wether it would be successful or not is up in the air but it’s goals are pretty solid.
So basically, a thing that will provide first-class capabilities only on Intel hardware, and won't be really optimised for maximum performance/expose all the underlying capabilities of the hardware elsewhere.