An ISA roadmap for x86 in general would be a godsend. I think the problem is that right now, Intel doesn't have a clear roadmap themselves, and kind of cobble it together as they go. AVX-512 was spearheaded by Xeon Phi, and then there's all the ML and more HPC stuff added along the way, and it seems like a great big mess at the end of the day. But beyond that, they seem to view ISA differentiation as a competitive advantage, and thus keep is secret till pretty much the last possible moment. It's kind of ironic since it gives AMD years to implement the same while the software catches up.
Would be even more helpful in reverse. Imagine how much better things would be if Intel could say, for example, "All processors released past 2025 will no longer support x87 or MMX", and actually hold to that plan. Though the ideal state would be Intel and AMD actually working together. Would better fit the reality of a world where ARM is a real competitive threat.