This is a theory I've had for some time but didn't have time to post before.
The Land of Departure's Forecourt has three chairs, and in Birth by Sleep we see Eraqus and Xehanort occupying two of them, with the middle one empty. If I remember correctly, this led some poeple to theorize they are part of a trio, but I'm not sure if that was the only reason. I believe the third person is their Master.
Some time ago, I read someone theorizing that Yen Sid is the third person in the trio because of him saying in BBS that he "used to walk the path of the Keyblade Master alongside Eraqus and Xehanort". Here is the link for the discussion, and here is the image. While that person interprets that line to mean "I trained together with Eraqus and Xehanort", I think it actually means just "I was a Keyblade Master just like them". After all, if Yen Sid was trained with Eraqus and Xehanort, you would expect Yen Sid to be mentioned in Xehanort's Reports when he talks about his youth.
I think this is what made me think about this hypothetical third person, and I thought that, going with the positioning of the empty chair, it makes more sense for the third person to be the Master, who we might see in KH3.
So, who is Eraqus and Xehanort's Master and how are them like? We know this Master trained them, passed the No Name to Xehanort, chose Eraqus as the Successor and probably passed the Master's Defender to him, and that the Keyblade War is the Master's favorite story.
First, considering all other trios in the series, I think the Master is a woman. The English version of Xehanort's Letter in BBS refers to the Master as male, but the Japanese text doesn't use any gendered term, so I think it might have just been the translators going "male as default". While we do know from interviews that the localization team communicates a lot with the Japanese staff, I doubt they asked Nomura about the Master's gender considering they had no reason to think the Master would even appear at any point. In fact, I think Nomura didn't even think much about the Master when mentioning them in BBS.
But it makes sense from how Nomura thinks for the Master to be a woman. Nomura loves parallels and connections, and there been many parallels and connections between the BBS characters and the Destiny trio, from how both Terra and Riku have history with darkness, to Sora and Ventus both having their friends as their power. Even further, Vanitas and Riku have similar dark suits and fight poses, Young Xehanort's fight stance is the same as Riku's, and actually Young Xehanort was even voiced by Riku's VA in BBS (at least in the English version, I don't know if we have confirmation about the Japanese version and I'm not an expert on VAs to try to recognize the voice). On connections, Riku and Kairi became Keyblade wielders because of Terra and Aqua, Ventus played a small role in Sora getting a Keyblade (like, the reason why Sora was a viable replacement to Riku at the start), if I'm remembering the interviews right, and is the explanation for Roxas's face.
Furthermore, I read some people saying Eraqus and Xehanort's chess game gave them Sora and Riku vibes (like, it's kinda like how Sora and Riku would always compete), and I honestly found young Eraqus similar to Sora when I first saw him. So, having the Master being a woman could parallel TAV in how Aqua is the only Master in the trio.
Now, let's try to guess the Master's name. We already have characters named Yen Sid, DiZ, and Eraqus. Eraqus is an anagram of Square, DiZ is partially a reference to Disney if I remember correctly that CoM Ultimania interview, and Yen Sid's name spelled backwards is Disn- Hey look, a coin!
So, I think the Master's name is Eni, which is an actual female name in Japan, starts with an "E" like Eraqus and the tooth of the Master's Defender, and it's "Enix" minus an X. I mean, why not? Eraqus's name only uses the Square part of Square Enix, so why not having a character referencing the Enix part of the name?
Now, something that I've been thinking for some time, what if we already know this Master? What if the Master is a character we have already seen? I guess you might be confused now, because, what character we know could fit the description I just theorized and be old enough to have trained Eraqus and Xehanort?
Actually, there IS one character who we have already seen, is female, and is old enough to be the Master: Kairi's Grandma.
Yes, that's my theory, Kairi's Grandma is Eraqus and Xehanort's Master.
Just think about it, the Keyblade War is "the Master's favorite story", a line Nomura made a point of including in the very first trailers: the E3 2014 and 2015 ones. See the E3 2014 teaser, all keywords are highlighted, and so is "Master", implying it's a keyword. And the Japanese version of that line is something like "I wonder how many times I heard about it from the Master". In other words, they highlighted the word "Master", implying it's important, in a line saying how the Master likes to talk about the Keyblade War. How many times have we seen Kairi's Grandma? Exactly three times: KH1, KHBBS, and KHχ's opening, and literally all three times she tells a fairy tale version of the Keyblade War, the same story they made sure to let us know the Master loves to tell.
Kairi's Grandma being important is something that would legit surprise people and would have the most subtle foreshadowing this franchise has had in a long time, so subtle most theorists didn't even notice. In fact, with my point above about parallels and connections, the Master being Kairi's Grandma would be genius. If my theory is correct, that might even be why they aren't showing Radiant Garden (because I honestly doubt they wouldn't make it playable): If the story is mainly about Kairi's Grandma (or, heck, mainly played as Kairi or having Kairi as the Party member, but basically Kairi-centric at all), then it would be a big spoiler, just like how they aren't including many scenes of TAV in trailers. Maybe Radiant Garden is even a late game world. After all, the person who trained the Big Bad showing up seems like something you would put in the late game.
If you still find this theory ridiculous, well, KHUχ is finally introducing blood connections, so doesn't it make sense to have the only known relative shown so far becoming someone important? In fact, giving Kairi's Grandma relevance could be what motivated Nomura to finally introduce another family in the franchise. Either that, or family is the next saga's theme, which would bring into question how Sora fits in since the entire franchise is his story, according to Nomura.