To the people asking why Incineroar would be the last reveal, there is a very, VERY good reason for it. Let's take it step by step for clarity's sake.
1. "Everyone is Here!" was decided from the beginning. From there, it is safe to assume that the fighter numbering was decided very early, especially since the character select screen itself is based on it.
2. Incineroar was officially revealed in 2016. The roster was decided in late 2015. Because of this, like Greninja before him, Incineroar was likely added later on and before that was just a placeholder.
3. Due to being added later on, it makes sense that Incineroar's reveal would be last: he was likely the last fighter decided on, significantly long after the other (potentially non-echo) fighters.
I'm certain the box theory is correct, that the final non-Ken character is Incineroar, and that this is why.
I'm not sure if I can agree with you on why Incineroar would be last.
Sonic and, as you said, Greninja were late additions and they were not the final reveals for Brawl and Smash 3DS & WIi U. Sonic was revealed in October, 2007, before King Dedede and Olimar. Greninja was revealed in April, 2014, and there were still plenty of unique characters left to reveal.
Incineroar could have been confirmed at any point after E3 2018. Why they decided to save him for last is something we can only speculate on.
See, I both very much love Isabelle's inclusion AND the shattering of that argument.
So it's like... extra Christmas for me.
My forecast only at this point includes Skull Kid and Isaac as potentials in the case that Geno isn't in.
I would be 100% cool with Isaac being an AT and Skull Kid being a boss in "Spirits". The game could release how it is and I'd be totally ecstatic.
Geno could be just a trophy in the game, I'm gonna main Inkling and be totally happy haha.
I just can't wait for the game. And you know, I'm excited to see how they handle DLC if they do.
I see it like this, they're either going to go all out on DLC or the base roster is it. I don't see an intermediate situation happening where our DLC is just promotional characters and third-parties; not that they won't be part of it mind you.
Sakurai knows that we're not going to get as many newcomers as we did the previous games due to factors like having a massive roster and also bringing literally every veteran back. To make up for this, he's going to make DLC an even more prominent feature of Ultimate than it was for Smash 4. This in itself sounds weird because with DLC, we are only getting newcomers but we also have to remember that DLC last game was a cramped period due to Nintendo needing a new Smash game ASAP. With Ultimate, we won't have that concern as the Switch's huge success and Nintendo promising to support it for longer than the usual Nintendo console allows for possibly years of Smash updates. I would not be surprised to see at least two years of DLC or a major expansion that you can also buy separately if you haven't got Ultimate yet for whatever reason.
Eight to twelve DLC characters would honestly not surprise me. It will likely be a mix of promotional characters, third-parties, and fan favorites.
Of course we still need to see who gets AT'd first, particularly for popular character. If said highly requested character isn't AT'd, they have a good shot at DLC even if they are essentially "dead" characters. That's because these could be characters Sakurai wanted to add but was unable to fit in the base game and now DLC gives him the chance to do so.