So here's my personal take on this whole situation.
I think the Growth Psynergy item does hint towards Isaac being in Smash. Obviously it's not a
true confirmation since franchises have had items represented in Smash without having characters represented before, (notably so with the Pitfall in Brawl, back before Villager was playable,) but the question that springs to my mind is: why did they wait this long to reveal it Showing it off would have taken ~10 seconds in the last Direct, so it hardly would have made a difference time wise. Also, given the circumstances in which it was revealed today, it's entirely possible that they didn't intend for it to get revealed yet
at all. This level of secrecy suggests to me that something much bigger is probably bundled with that item, and the logical conclusion is that Isaac is in the game.
However, while this is certainly a point in the leak's favor, it's far from a confirmation that the leak is legitimate. After all,
Isaac could be in the game and the leak could be fake at the same time, and that's what I personally believe at the moment. I can't overlook the inconsistencies between the leaked empty banner and the E3 version; they just don't add up.
Here's the comparison between the two.
And here's a comparison between the E3 empty banner and the current filled banner for reference. (Excuse the fact that the E3 banner is ever so slightly zoomed in.)
The banner doesn't change at all from the E3 version to the current version from what we can observe. While it's entirely possible that the background is changing behind the characters, that would be quite bizarre since the background is likely one layer and the characters on top of it are likely just resized. However, the banner changes quite drastically in the leak in the spots that are normally obscured by characters, and
despite supposedly being the final version of the banner, the leaked version notably and randomly gets less detailed. I can't fathom a scenario where someone thought it would be best to remove an entire island from the final version of the banner even though it remains unseeable behind the characters. The only logical explanation for this as far as I'm aware is that someone edited these characters out manually and then redrew the background in the spots where the characters were edited out. If you accept this, you accept the leak is fake.
With that out of the way, I think the remaining newcomers would be Isaac/Incineroar as true newcomers, (since Incineroar is just as likely as it was before if the Grinch leak is fake,) and Ken/Shadow as echoes. Even if Grinch leak is fake, if we assume Isaac is in, then it's highly likely that box theory is dead, and if so, Shadow starts to make a lot of sense again, at least to me. Finally, I think the Square Enix character is still going to be the first DLC, whether it be Sora or Geno, or even someone else. (Realistically though, I think it's one of the former two.)
