Oh boy
The theory is that...
There's 3 missing slots in the music menu.
What this may be potentially implying, is that there will be 3 new franchises added to the game.
Not third parties necessarily-went and edited that.
I'd bin this, except...Nintendo and Sakurai is totally fine with hiding this stuff right in plain sight. Remember the box theory?
So...give it some thought.
I'm pretty sure that this Music Theory 2.0 means nothing.
First of all, of course its not a perfect square. It's for the same reason the CSS or stage select screen isn't one - most numbers don't form perfect squares or rectangles. I doubt they're thinking "we need to add exactly three franchise to form a perfect rectangle, no more no less".
Second of all, the box theory had something else major going for it, which was that it seemed to have accommodated for Ken. If it weren't for Verg leaking Incineroar as well (and the fact they the probably wouldn't have ended the reveals with just a single echo fighter), the box theory could have been used to argue that there's 1-2 characters left rather then 2, since it wouldn't have had to have been completely filled in.
Finally there's that Ultimate, in it's current form, is meant to be a complete product. It'd be strange for them to design their UI around DLC that possible hadn't even been fully finalized rather then designing it around its current state. Of course they'd still want to make it easy to accommodate for DLC, but I still don't think they thought "hey lets leave out exactly three open spots for the DLC just in case".
Even if we do get exactly three new franchises I won't buy this. It'd probably just be a very possible coincidence.