In regard to tomorrow's Direct, my bet is that we will be seeing Skull Kid.
I recently went through and read all of loz18's GameFAQs posts, and especially after hearing about how little credibility he has in so many's eyes and how he supposedly got a lot wrong, I was expecting an accurate leak of the E3 presentation's info in the early posts and pretty much nothing else that has held up or seems likely to do so. I was extremely surprised when I instead found a wealth of information that was either exactly correct or very close to correct, with the few inaccuracies being quite believable and understandable mistakes. From his descriptions of his playtime, it is obvious that he legitimately played a limited beta build that would normally only be available to Nintendo and Sora employees, featuring about 12 characters and the placeholder logo from the Inkling teaser rather than the E3 demo. It doesn't sound like he works for Nintendo, so the "interesting people" he knows must be miracle workers to get him anywhere near a build of Ultimate before the game's official unveiling. What is clear is that his connection definitely works for Nintendo/Sora, though, as since the beginning he's been mentioning how a lot of his information comes from assumptions based on materials that are collected or seen by his friend either for or directly in relation to the game and/or its marketing, not to mention his source shared more details than any build of the game or demo would reveal.
I know its an unpopular opinion in the community these days to trust loz18, what with his odd behavior on Twitter, but just like when everybody was saying the Gematsu leak was fake due to its partial inaccuracies, I personally still have faith in everything he said while on GameFAQs. Thus, I expect that Spirits Mode will be the main way to unlock characters, that when we finally hear more about costumes like Meta Ridley that Zelda will have a costume based on her BotW appearance, and that Skull Kid has a significant role in the game, most likely as a playable character.
Admittedly, however, I have much less faith in what he's said since moving to Twitter. It sounds like his friend may have seen material that implies that implies that Isaac is playable, but it could very well be a more far-reaching guess than all of the previous ones the two made. That said, however, I would put personally consider DasVergeben to be about as credible as loz18. Unlike loz18, who leaked many accurate or nearly accurate details, in reagard to SSBU, Vergeben has yet to predict anything that would undeniably prove that he has accurate inside information on the game. If we get Minecraft content or Incineroar as a playable character, I'll trust him as much as early loz18, but until then he is undoubtedly the lesser of the two in terms of credibility, especially if you choose to focus only on loz18's time on GameFAQs.
There is an astounding amount of misinformation out there surrounding what loz18 said before moving to Twitter, both among fans and even in the loz18 leaks' articles, threads, and screenshot galleries across the Internet. Everything I've said here, however, is backed up by a thorough reading of dozens of his posts, so I offer you my assurance that it is all accurate to the best of my knowledge. That said, if you have any questions or disagreements about the veracity of my interpretation of events, I will do my best to link you to the relevant threads on GameFAQs so that you can see the original sources for yourself.