Whenever I would post tidbits about this, I got a couple of requests to make this its own thread, so I decided to go ahead go through with it.
So, the boxing ring stage. You've seen it, I'm sure. Some people aren't exactly sure if it's a Punch Out related stage, if it's a Kirby themed stage, or just a generic new boxing ring stage just for kicks. Well, I'm gonna tell you that this stage is a Punch Out themed stage for a multitude of reasons. So let's begin I guess...
First off, the audience is human, so you can rule out any possibility that this is a Kirby themed stage (all Kirby boxing rings have Kirby characters instead). The marquee's in the background are all dot styled, which is consistent with the marquees in Punch Out Wii, and even the title screen and round indicators for Mike Tyson's Punch Out.
Various dot styled marquee screens:
http://i.imgur.com/sgmo0ru.png
All logos in Punch Out Wii are adorned with clusters of stars, which are usually in 3's because triple star punches are the most powerful punch in the game, as well as the "leaf vines" on the sides (forgive me; I don't know the term for this) and a giant star somewhere in the middle. The logo in the ring, while being smash themed, shares all of these characteristics.
3 star cluster on bottom:
http://i.imgur.com/QSCAXpe.jpg
Minor/Major/World Circuit Emblems:
http://i.imgur.com/oWzpteX.jpg
The ring ropes are also identical in both color and configuration as the Major Circuit ring in Punch Out Wii. Similarly, the corners are all identical colors and positions as the Punch Out Wii corners. The logo in Smash also features a ribbon at the top, which is a direct throwback to the World Circuit ribbon at the top and bottom of the logo.
Major Circuit ropes and World Circuit ribbon:
http://i.imgur.com/UV4O3mB.png
Also, as a random tidbit, the ribbon in the Smash Punch Out stage says "CHAMPIONSHIP" on it, which is probably because if it straight up said "Major Circuit" on it, it would be flat out obvious. Plus since Smash Bros doesn't have "circuits", championship makes more sense in the context of the game.
The final fight in Punch Out Wii, Mr. Sandman, has a very particular entrance for both his Title Defense and Contender fights. He approaches the ring while there's a green laser light show that goes on behind him. Well, check this out...
Sandman intro:
http://i.imgur.com/hvnTGDl.png
Mario in the ring:
http://i.imgur.com/Odu2I7I.jpg
Same green laser show.
But yeah, there are an abundance of intentional details that suggest this stage is designed to be a Punch Out stage. There may be a few details that I'm missing, but those were all the details I seemed to notice. Unsure if this confirms Mac being a character, but at the very least there's a stage from the Punch Out series in the game.