If you want to build a personal computer, you'll probably need to make it Windows compatible. If not for other reasons, because users actually want Windows software. With that restriction in place, it's very hard to be really innovative.
Multiprocessors only became mainstream when Windows XP became mainstream. I've used two and four CPU machines under NT and Unixes, but they would hardly be mainstream (even if they were not much more expensive than top-of-the-line single-processor PCs). And, under Windows, a dual 200 MHz machine was much smoother than a single 400 MHz one.
The same goes for 64-bit machines. On the Unix side of the fence (which was also the RISC side), 64 bits were useful mostly for the extra memory. Having more than 4 gigs of RAM was outrageous until relatively recent times and the demand for larger address spaces was somewhat smaller than the one for more processing power but, again, it wasn't until Windows XP started supporting them that AMD64 processors became common. And the only reason it didn't follow the Itanium family is because it could pretend to be a 32-bit x86 well enough to fool Windows-based software.
Before we get innovative computers like we did in the 80's and early 90's, we need to get rid of the assumption a PC is a machine that runs Windows.