I have not yet seen a thread like this, so I figured it was okay to make one myself. I would like to share my theory/theories for KH3's Final Boss Battle and hear yours, too. Be sure to put said theories in spoiler tags. It's more dramatic that way.
I personally think that Xehanort will NOT be the Final Boss in KH3. That would just be too predictable. He will be the SEMI-Final Boss, much like Barthandelus from FFXIII. Instead, a twist will occur where one of the heroes usurps his role as antagonist, "disposes" of him (ironic, given his treatment of others), and attempts to claim Kingdom Hearts' power for him/herself. This person will not be corrupted by Darkness (like the ten trillion other Bosses in the series), but will instead try to DESTROY Darkness and remake the universe into pure-Light, believing that Darkness really was the source of evil and misery all along, and that Xehanort's actions only reinforced that idea. The other heroes will try to talk this person down, but cannot ease his/her paranoia against Darkness, and will ultimately have no choice but to face this person in order to protect the balance of Light/Darkness. However, this can't just be ANY one of the heroes. After all, nearly all of them have already served as Bosses in some way/shape/form at some point in the series. But there is one playable, series-original character who has remained a hero all the way through. If you can guess who I'm talking about, I'll continue my theory.