How about a re-completed Terra, from the future? Seeing as most of the super secret bosses you face in the Kingdom Hearts games have something to do with Xehanort/Xemnas, the Lingering Stagnent, this my guess. I mean "Terra's" Nobody (Xemnas) was defeated, and his 'heartless" destroyed, what are the chances they some how reunited and become a newly formed Terra.
I mean they are the same height, and has a similar fighting style Xemnas. And maybe because he was reborned in such a way, he wasn't able to reobtain his keyblade, but those ethereal blades instead. As well as being able to wield both light and darkness of his heart through his experiences as that? Also why would do you think it would be hardest fighting it with Terra? I mean you would be fighting the you from the future who knows all your moves? I don't I may be, and most likely will be, wrong but that's my guess
