How to make an instruction 15 bytes?

Question

assembler

The maximum command length in the architectures specified in the tags is 15 bytes. Somewhere on Habré (I lost the article) I read that with the help of prefixes, you can create a 15-byte valid nop. Is it possible to make any command 15 bytes in the same way? Is there a limit to the set of valid prefixes, or can I just add a couple of random bytes to the beginning of the command?
here ref.x86asm.net/index.html#column_pf says that the prefix is

Fixed extraordinary prefix, which may change the semantic of the Primary Opcode. Usually used in case of waiting x87 FPU instructions, and many SSE instructions. F390 PAUSE, 9BD9/7 FSTCW, F30F10 MOVSS

Is there a set of prefixes that do not change the behavior of any commands?
Can I just add a couple of random bytes to the beginning of the opcode and get a valid command with the same behavior?

Answer 1

Assembler must be concise in order to run efficiently on any hardware. In addition, given the development of new architectures, it should still be promising (see x64 implementation). I don't think there is a duplicate command/prefix - every option counts.
Something does not suit single-byte nop?