This idea from @Ronny Martinez  it is very interesting you can easily swap quad or  wheels from biped body with snaps. Remind me of your idea @Piers Kennedy  for omni but you got to bottom instead which I like it too.

The difference here is that switch won't be able to be on the back it is on the way, so we have to agree on what path to take

@Bill SCHONFELDER  see this approach, can you design standard universal joints that we always use for this kind of expansions/modules?