Throughout the final mix remakes of the KH games there is added content, like extra boss battles and mini quests and collectables. What if in Kingdom Hearts 3, there is mention to these add-ons and are counted canon to the story? Like for example the Lingering Will battle in KH2FM, what if when the image of Terra in his armor is brought up by either Aqua or Ventus, and Sora, Donald and Goofy are like, yo! We beat a guy like that one time! Lol I know it's far-fetched but it's still something to think about

Yes, the final mix content is canon to the story. For example, the Unknown in KH1FM, who turned out to be Xemnas.

As KeybladeLordSora said, FM content is canon. Also, optional keyblades are canon. Not joking.

I don't think secret bosses really count as canon as in KH2 it is never brought up that Sora fought Sephiroth and Sora doesn't recognize Sephiroth in KH2.  The only canon moment from a secret boss I can think of is when Xemnas mentions that he has been to see Sora, which he does in the boss fight; but even then there is no mention that Sora ever encountered Xemnas.  I would say that anything post-game is non-canon and is more of foreshadowing to characters who will appear in the future.  (Unkown = Xemnas, Lingering Will = Terra, Mysterious FIgure = Young Master Xehanort)

I don't think secret bosses really count as canon as in KH2 it is never brought up that Sora fought Sephiroth and Sora doesn't recognize Sephiroth in KH2.  The only canon moment from a secret boss I can think of is when Xemnas mentions that he has been to see Sora, which he does in the boss fight; but even then there is no mention that Sora ever encountered Xemnas.  I would say that anything post-game is non-canon and is more of foreshadowing to characters who will appear in the future.  (Unkown = Xemnas, Lingering Will = Terra, Mysterious FIgure = Young Master Xehanort)

Glad your following what I'm trying to say^^

The simplest way is to say that some are, some aren't. I'm gonna go with they're canon until confirmed otherwise (KH1 Sephiroth).

Some boss fights are and some aren't. Sephiroth in KH1 was not canon but Sephiroth in KH2 might be. Kurt Zisa and the Phantom were just extremely strong heartless so one has no reason to think they would not be canon. Dustflier in Days was also just an extremely strong heartless.

Also the Cups appear to be canon ( RE: Coded shows/proves this later) 

 

Unknown( Xemnas) in KH1FM was definitely canon.( Nomura also confirmed btw)  Xemnas references that fight at the start of KH2, using the same phrase he said to Sora in that fight.In Days, he tells Roxas "I've been to see him" Even dreams were pieces of memories and connections to Sora

 

LW/LS and Mysterious Figure are also canon 

 

Here is Nomura talking about some of this stuff and other boss fights in BBS

• This new character (Mysterious Figure) must be someone involved in the story, right?

I think it’d seem so, but at the point in time of Birth by Sleep, this person’s true figure isn’t fully understood. Similar to the way the Lingering Spirit was, it’s a character who at the present time is an existence of mystery.

 

• Talking about the FM bosses

There are many subtle alterations added here and there between No Heart’s armor and Xemnas’s armor in KH2. They couldn’t use the giant cape from Xemnas’s armor this time. Xemnas stayed seated, but No Heart stands up somewhere along the way, so be careful. After standing up, No Heart moves around a lot.
The staff originally said that they wanted to make No Heart and Armor of the Master opponents with cutscenes like the Mysterious Figure, but it was rejected in order to not take away from the consistency of the plot. Their strength and abilities are the same as the real ones, but the two in armor are handled entirely as revived data, and exist separately from the original plot.
If the bosses were to be ranked according to strength, it would go: Vanitas’s Spirit

 

• Talking about Mysterious Figure (pre-release)
   The new secret boss will be like how Lingering Sentiment was to KH2FM. You will think it’s impossible that that can happen at that time of the story, but it actually happens

 

Final Mix stuff does tend to add not only to gameplay but also to plot.

i'll just assume a Final Mix add on is non-cannon until it's mentioned or hinted at in another game. 

Indeed,  the content in the Final Mix versions of the games, where secret bosses are concerned, have and are indeed canon to the series, like Unknown and Lingering Will! (Those are the only two secret bosses canon to the story, right?  Or do Mysterious Figure, Armor Of The Master and Vanitas's Lingering Sentiment count as well?)  The additional cutscenes added also extend the lore of the series, so that counts too!  I just love that the Final Mixes do that, ya know?  It's like an Extended Edition of the games!

Edited October 31, 20159 yr by The Transcendent Key

Unknown, Lingering Will, and Mysterious Figure are all canon. They ar ementioned in other games and their apperance is tied to the overall plot of the story.