I know there are already a number of extensions specified in the specifications, such that Risc-V could be relevant to design the simplest of microcontroller up to the most powerful super computer. I suppose it is possible and allowed to design a CPU with proprietary extensions. What should prevent an ARM type of situation is the fact that so many use-cases are already covered by the open specifications. What is not there yet, to my knowledge, are things like graphics, video, neural-net acceleration.