This may sound absurd, but it was a mental breakthrough for me to realize that all these "what-ifs" (multicore 6502 computers or whatever) are really simple to

This may sound absurd, but it was a mental breakthrough for me to realize that all these "what-ifs" (multicore 6502 computers or whatever) are really simple to simulate in software only, with a bespoke simulator, WAY above register transfer level. There is really no need to deal with the peculiarities of FPGAs or Verilog; just write an instruction-level simulator for the ISA you choose (that's really simple for the 6502), make it run several instances for multicore architectures, and there you go for your custom computer. I mean this as a hint for people who are similarly stuck with RTL tinkering when they actually want to tinker with system architecture.