amd64 isn't going away any time soon. Intel is still the largest player here. If I were Intel right now, I'd be trying to come up with the next iteration of x86, probably 256 bit.
There don’t seem to be many advantages to words larger that 64 IMO (they do exist, but things like GMP exist for that). If the purpose would be parallel processing (a la SIMD), we already have “256 bit x86” with AVX2. IIRC, Intel’s x86 chips can process 256 bits of integer data at the same time. And if you want 512 bits, AVX-512 exists as well (but it’s implemented internally as two sequences of 256 bit data)