What will a given CPU architecture do in the event that we were to invert all of the instruction bits to a given instruction?
If you are decoding instructions using logic based on individual bits in the opcode, this &quot;inversion' scheme is probably counter productive.  If the opcodes are decoded using a simple table lookup of the &quot;value&quot; of the opcode, then the actual pattern of the opcode is just arbitrary and the inversion scheme is just as valid as one based on the alphabetic order of the mnemonics.
For stack based machine, variable length opcodes could be more valuable, shorter opcodes for push, pop, and any immediate value instructions to give more space for address or data bits.
Self-modifying code has been out of fashion for decades, so being able to flip the meaning by inverting the bits would seem to have no utility.