My thoughts on the whole Square Enix rep? Vergeben isn't sure because the Square Enix was decided on later, as it is a DLC character.
Mark my words, Box Theory or not: Ken and Incineroar will be the final two characters featured in the base game. You may not like this, but hear me out. Not only does it fit with Sakurai's 'there will be less newcomers this time around', but also fits recent Nintendo Direct history.
In both 2016 and 2017, the final Nintendo Direct not directly related to a particular game was in September: on September 1st for 2016 (3DS Direct) and September 13th for 2017 (Nintendo Direct). After those, we got three Nintendo Direct Specials in 2016 (of which two were Japan-only) and two in 2017. Almost all Nintendo Direct Specials feature a game that launches around a month after that particular Direct and the same was true for four of the five mentioned Direct Specials in late 2016/2017 (exception being the Animal Crossing Direct in 2016).
Assuming the same is true for 2018, we've had our final non-Special Direct, and looking at the calendar of larger upcoming titles, I'm guessing there will be a Pokémon Let's Go Pikachu/Eevee Direct mid October and then an early November Direct for Smash Ultimate.
While Incineroar is a Pokémon, I doubt we'll see a Smash announcement in the potential LGPE Direct: it may just overshadow the entire thing - not to mention, Incineroar has little business in a Kanto-oriented game.
Which just leaves us with the Smash Direct in November. Especially considering Sakurai has stated he wishes to announce all characters before launch (which was also true for Smash 4 Wii U by the way, Duck Hunt's video was shown one month before the Japanese release of the game), it feels like it'd make sense to finalize all character announcements in that particular Direct. Keep in mind, the final Smash Direct before Smash 4 3DS also introduced Charizard and Greninja, the final before Smash 4 Wii U only showed Bowser Jr., and both focused more on the many modes of the games. With the alleged Spirits mode still under wraps, I'm guessing the same is also true for Ultimate.
Now, if we were to go a bit crazy, we may just see the Square rep talked about as upcoming DLC (like how Mewtwo was talked about at the end of the 50 Fact Extravaganza) - but I seriously doubt wel'll see anything more included.
I'd love to be proven wrong and it might just mean the Smash Ultimate hype train ends on a bit of a wet fart, but I'm going to play it 'safe' and assume these are our final characters. 72 is an obscene amount anyway - more than I could have ever imagined.