"There's always a bit of truth in legends." - Ahsoka Tano
By now, I'm sure everyone's aware of the aforementioned loaded quote from the season finale of Star Wars Rebels, and you've probably come up with your own ideas about what it means. Personally, I took it as an homage to the old expanded universe, a way of saying "just because the story's going in a different direction, it doesn't mean everything's been forgotten;" I didn't think there was necessarily anything in particular about Legends content that Filoni & Co. were referring to...that is until I saw the following on Twitter:
Yes, I know Darth Bane is canon and, therefore, not Legends, but the majority of his story is Legends and I think that's what has the potential to be expanded upon in Rebels. In fact, I think whatever legend he's referring to is going to have something to do with the Sith Holocron that we see Ezra open as season two comes to a close, so why not the "father of the modern Sith," so to speak? I think it stands to reason that an ancient Sith temple would house the knowledge of an ancient Sith Lord, don't you? Further, I think it makes sense that Maul would be there looking for guidance from the creator of the master and apprentice system since his apprenticeship was cut short and, ultimately, even though he led deathwatch and his band of criminals, he's still someone who relishes a master's approval - you see it in Son of Dathomir, even though he's been obsessed with revenge against Obi-wan and the master who abandoned him since he was literally made half the man he used to be, he still yearns for Palpatine's approval. And, I mean, you saw how easily his first apprentice, Savage, was dispatched by his former master, so I imagine he'd be searching for whatever knowledge he could find to keep that from happening again with his next apprentice (which could still be Ezra, just saying). It would certainly make for an interesting storyline, though, if Ezra learns about the power of the Sith and the rule of two and tries to convince Kanan that they should become a master and apprentice like that instead of like the Jedi...
So that's my take on it, and, yes, I'm aware that I'm ignoring the most popular theory that the book pictured is Heir to the Empire and that means Thrawn is clearly who we'll be seeing show up in Rebels...I just don't buy it. For starters, I don't think you can prove that the book shown is actually HttE, nor do I care if it is because Dave Filoni is a pretty coy fella and would do something like that on purpose to throw people off. Additionally, I think Thrawn's character has already been replaced by the white-clad, caped, supposed "Director" in Rogue One. And, lastly, based on where the story is headed, Ezra is going to talk to someone in that holocron, and it's not going to be Palpatine, Vader, Dooku, or Maul, so it has to be a more legendary Sith...just has to. So, who do you think Filoni is referring to? Do you agree it'll be one of the Sith Lords I mentioned? Am I an idiot and it's obviously Thrawn? Or do you think it's someone completely different? Let me know in the comments below, and I'll catch you guys next time 'round...and, you know, may the Force be with you...