I agree with @benaadams here, The EVM as a whole is very much U256 based, so abstracting down to riscv would decrease overall execution performance. From my limited STARK knowledge, I know that the AIR can be built more easily over register-based instruction sets.
The immediate first solution I think of is using a language like Cairo, or a pseudo-cpu architecture similar to risc-v but designed for emulation, e.g 256 bit registers. These solutions are somewhat adhoc and may present overt complexity when actually being implemented, but they may serve as an alternative POV.