For what? The simplest possible ISA? Something like an OISC or a ZISC, probably. Simplest "virtual computer"? Maybe the SK(I) combinator system? Specific improvements to this architecture? My personal preference would be to ditch the three arguments that aren't always used, have two 8bit instructions per 16bit word, use a stack (to eliminate the operands). But that's just one mode of thought for a very simple computer, not the only one. This line of thought is particularly inspired by the F18 Forth chips. They're quite minimal, simple, tight CPU designs: https://www.greenarraychips.com/home/documents/greg/DB001-22...