There's not a strong case for redoing the RISC-V encoding with a new RISC-VI unless they run out of 32-bit encoding space outright, due to e.g. extensive new vector-like or AI-like instructions. And then they could free up a huge amount of encoding space trivially by moving to a 2-address format throughout with Rd=Rs1 and using a simple instruction fusion approach MOV Rd ← Rs1; OP Rd ← etc. for the former 3-address case.

(Any instruction that can be similarly rephrased as a composition of more restricted elementary instructions is also a candidate for this macro-insn approach.)