Square, octagon's never ever gonna happen. Mayweather's only getting pulled back in because this is light work for a lot of money.
McGregor's going to be a big underdog and will get a lot of recreational bets on him at +800 to +1000, it's probably going to be fairly close odds between Mayweather decision and Mayweather KO, and honestly, the outcome is more or less up to Mayweather. If he wants to put on an exhibition and gets his normal unanimous decision and dance around McGregor all night, he can. If he wants to really try and pour it on...even though he's not carrying much power (for nearly his entire run at welterweight, if we're being honest), he's going to be able to hit McGregor at will, I think.