So, 40 year old x86 and 30 year old ARM is not full of legacy issues and layers over layers of hacks?
A good fresh start has always possibilities, real question is if Apple is competent enough to pull it off.
Other than booting it, and EFI is addressing that, it's a fairly clean and nice architecture. Some of the registers have legacy names. Are there any really odd legacy behaviors?
x86 is a variable-length encoding, which is a good thing for cache use, but it doesn't help these days because the most common instructions are not the shortest, and the shortest ones are things like BCD and BOUND that nobody uses.
x86-32 doesn't have nearly enough register names, causing totally unnecessary memory spills, which the hardware was never good enough to hide, and having to pass arguments on the stack. That's why x86-64 is faster than -32, even though 64bit wastes so much cache space.
And some instructions are just randomly slow because of handling the weird encoding, like 16-bit math is slower than either 8-bit or 32-bit math.
Then there's eflags, but that's a minor complaint.