"What would the TempleOS of CPUs look like, I wonder?"

In the old days, probably Burroughs B5000 for it being high-level with security and programming in large support built in before those were a thing. If about hackability, the LISP machines like Genera OS that you still can't quite imitate with modern stuff. If a correct CPU and you want it today, then I've been recommending people build on VAMP that was formally-verified for correctness in Verisoft project. Just make it multicore, 64-bit, and with microcode. Once that's done, use the AAMP7G techniques for verifying firmware and priveleged assembly on top of it. Additionally, microcode will make it easier to turn it into a higher-level CPU for cathedral-style CPU+OS combos such as Oberon, a LISP/Scheme machine, or Smalltalk machines. Heck, maybe even native support for Haskell runtime for House OS.

https://www.smecc.org/The%20Architecture%20%20of%20the%20Bur...

http://www.symbolics-dks.com/Genera-why-1.htm

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.217...

http://www.ccs.neu.edu/home/pete/acl206/slides/hardin.pdf

http://www.projectoberon.com