Sin SIN sin!

I'd love to see Jecht's Aeon as a secret boss. I've said for a while that it'd be a cool secret boss to go to Destiny Islands and find Tidus panicked saying that the darkness got his dad then going to find his Aeon and fighting it.

Adult Tidus!

 

None of them .

Probably Jecht final form. Doing non-humanoid bosses is always tricky (and ends up being some of their more unnecessarily challenging fights) but I'd like to see what they could do.

Jecht but I'd prefer it in his human form

Jecht in the Hercules Colisseum would be a great spectacle. I can see him gloating about being the best in the KH universe in front of his fans with his amusing ego.  Would LOVE to see Jecht and Hercules going hand to hand in a match and him teasing poor Sora. Of course, a battle againts him is a must in this case.

 

I can see it happen, and it would be truly enjoyable.

I actually REALLY like Final Fantasy X (3rd favorite game of all time), but nobody in it really scream KH Secret Boss. The main cast is great, but what reason is there for them to fight Sora? Also, while the villains serve the FFX plot well, none of them leave as strong an impression as other Final Fantasy villains (Sephiroth, Kefka, etc.). Heck, wasn't that logic used for Dissidia by making Jecht the "villain" for Tidus?
 

 

Jecht in the Hercules Colisseum would be a great spectacle. I can see him gloating about being the best in the KH universe in front of his fans with his amusing ego.  Would LOVE to see Jecht and Hercules going hand to hand in a match and him teasing poor Sora. Of course, a battle againts him is a must in this case.

 

I can see it happen, and it would be truly enjoyable.

I actually really like this idea a lot!

Shuyin is an underrated villain. I'd love to see him

I'd have to say Adult Tidus. I'd like to get some payback for him beating me sometimes in KH1. Shuyin wouldn't be a bad secret boss either.

Hmm, there are many good choices here to choose from! But I think I'll go with Sir Jecht in his Final Form! Getting to fight that monstrosity would be insane! Just imagine all the chaos onscreen! And he's never once been mentioned in the KH series, since Tidus is there, even though only for KH1, a bit of Re:Coded and a bit of Re:Chain!

 

Other secret bosses that I would love would be Dark Aeons, since they represent dark versions of summons, hence which would be badass to fight against, and Seymour! I love the music playing in the final battle against him, and getting to fight him would be awesome! X_X

None of them, because I don't like anyone from X aside from Yuna.