I'm still not convinced of the arena's origin as a Punch-Out stage. Every other stage that has been shown so far (save this one that is likely a part of a single player mode and not a normal stage) has had its origin been immediately obvious, by either designed to be instantly recognizable or just flat out using uprezzed assets from the source. The arena's supposed connections to Punch-Out are a lot more tenuous (and many of the cited examples aren't exactly Punch-Out specific).
It just seems strange to me that they would design such an unspecific stage for Punch-Out, especially when it just had a major console release a few years ago to pull designs more closely from as has been the case for all other stages so far.
I don't know, there's just this feeling in the back of my mind that keeps telling me it's a generic Smash Bros. stage. Which, if that were the case, would be pretty bad news for Little Mac.
"Smash Circuit" is a dead giveaway.