If you are to develop a CPU with an ISA level security proof, you need a homogenous system.
Something like a simplest MCU with few registers and hardwired instructions, no pipeline or OOOE, and flat memory that can be 100% mathematically verificated.
The Transputer T800 was formally verified. It would also be a great candidate vor hardware isolation, with different processes running on different CPUs, linked via the Transputer's hardware channels.
So you are talking about a special purpose processor solely for the purpose of being 100% deterministic? It'll be slow, but potentially more secure than its general purpose counterparts. Which of both would be more reliable security-wise? If you put both in a single system respecting the purposes, a heterogeneous system is the result.
You will still get something very different from HSA's understanding of heterogeneous design or secure enclave processors (although those ones come closer to the idea).
The thing that is approaching it the most is a smartcard CPU. Flat X-i-P rom, cacheless, embedded mram or fram, <100 instructions, full register state determinism.
If you are to develop a CPU with an ISA level security proof, you need a homogenous system.
Something like a simplest MCU with few registers and hardwired instructions, no pipeline or OOOE, and flat memory that can be 100% mathematically verificated.