Played some good Punch-Out yesterday, thought about how Mario was a referee that one time, thought about some fanfic I was reading recently, and it reminded me to share my newest headcanon, which I wish I thought of fully on my own. Punch-Out and Super Mario Bros exist in the same universe.
Not only that, but since Lil Mac's also a New York native just as I imagine the Mario Bros are (though instead of being from Brooklyn, Lil Mac comes straight from the Bronx), I headcanon that Mario and Luigi met Little Mac early in their lives before they became Mushroom Kingdom heroes. At the time, Mac was still an amateur and hadn't gone pro yet. He immediately became one of the Bro's earliest and closest pals, and they often sparred with him before he found Doc Louis. They attended all his early matches, and Mario stepped in for the referee for just one match when the normal referee fell ill. While they don't see each other often anymore because of the adventures, the Bros still consider Mac one of the most important people of their early lives.