I'm not a chip designer either, but IIRC, the current architectures are actually something similar, whereas the CISC is broken down into custom RISC-ish instructions by a HW layer, which is then executed by the RISC-ish cores.

And while it might be doable (x64 and ARM on the same chip), I think it might not be a smart decision, since some things will not function correctly or will have to be redone (e.g. speculative execution).

I don't know either architectures on a deep understanding level (x64 or ARM), but I have a feeling that there should be some big architectural changes that would prevent x64 code to run on ARM natively.