Ganondorf's return in The Legend of Zelda: Tears of the Kingdom is bringing back one of the wildest lore theories from Breath of the Wild, and it all comes down to a tiny piece of tapestry.
In Breath of the Wild, there's a tapestry that depicts events 10,000 years prior to the start of the game, where an ancient princess and hero battle alongside the Divine Beasts numerous others to seal away Calamity Ganon. The tapestry appears in Impa's house in Kakariko Village, and it figures into Kass's song about Ganon's ancient defeat.
Since the series usually revolves around different incarnations of Zelda and Link, the blonde princess and Master Sword-wielding hero in the tapestry seemingly makes it obvious ancient versions of the two heroes were working together to defeat Ganon. There's just one problem: why is the hair of this ancient "Link" red? Why does he have an extended nose? Why is his skin green and blue? Actually… Why does he look so much like Ganon?
The idea that Ganon would be fighting another form of Ganon is pretty goofy on its face, and it's broadly been accepted that the tapestry just takes some creative liberty with Link's appearance. But now that we've gotten a very Ganon-centric Tears of the Kingdom trailer, the floodgates of debate are opening themselves back up.
Admittedly, the theory does pretty much come down to "that guy looks like Ganon," so there's not much to pick apart here - and in fact, the game's discussion forums have been so clogged by these hot takes that pretty much everyone is getting sick of it.
There are two things worth bringing up, though. First, the Zelda in the tapestry has somewhat darker skin than how we typically see the princess depicted, and we see a new character with similar skin tone in the TotK trailer wearing the same clothes as Zelda. Second, the Zelda we're familiar with has an apparent ally in TotK, one who has some visual similarities to previous depictions of Ganondorf.
Personally, I'm extremely doubtful about the 'ancient hero Ganon' idea, but all the Zonai references we've seen in the official Tears of the Kingdom previews we've seen so far do suggest that the new game will be diving back into a lot of the deep lore about the ancient world established in Breath of the Wild. I wouldn't be surprised if we do learn a good bit more about the ancient princess and hero in the end, but at this point we've barely even got enough to start speculating on.
Of course, the real question remains "can we play as Zelda?"