That's a very hard question. First of all, it depends on what your goal is. Do you want to create a high-performance general-purpose CPU? Do you want to create

That's a very hard question. First of all, it depends on what your goal is. Do you want to create a high-performance general-purpose CPU? Do you want to create a power-efficient CPU? Maybe a special-purpose accelerator? How about a low-cost microcontroller? Second of all, none of those above are directly dictated by the ISA, but rather by the hardware implementation (think AMD Zen vs Intel Core or the embedded ARM Cortex M4 vs higher-performance Cortex-A53). An ISA can however more easily accommodate certain goals (RISC-V for example is very easily extensible, so if you want to create a special-purpose CPU, it might be right up your alley). Finally, software also matters a lot. Compiler quality is very important in the resulting performance. I think mediocre compiler support and insufficiently advanced compilers are one of the reasons Intel Itanium never took off, for example. Itanium was a very complex ISA and therefore harder to exploit to its full potential.