I find the notion of Xehanort's tale to be the alluring and meritoriously beneficial to the game's continuity. Playing as the series' antagonist is a nice refresher. It excites me of all the limitless possibilities we could have. Identity Crisis? Sporadic memories from both Terra and Master Xehanort permeating through his mind-unaware of the origins of his existence. Struggling with two voices whispering in his head; one deceptively seduces him closer to darkness while the other boldly tries to bring him to light.
Possibly associating with Ienzo, Dilan, Braig, Even, Aeleus, Isa, and Lea also spikes my interest. Seeing how Organization XIII comes to be brings my interest to a new forefront. The abduction of the citizens of Radiant Garden for their heinous experiments...it all just makes me writhe with anticipation. Seriously, who does not yearn to see the fall of Radiant Garden? How Xehanort banishes Ansem the Wise to the Realm of Nothingness? Him finding "The Door" and perhaps first summoning his Keyblade? It would contribute a tremendous deal to the story in my opinion. He may Remember Aqua and Ven from his past as Terra and begin a search for them, which could potentially push him further into Darkness. In conjunction with thishe can remember his past as Master Xehanort and remember the seven hearts of pure light thus would spur his search to find them later on. These memories could perhaps be evenly distributed among Ansem Seeker of Darkness and Xemnas. Obviously, this impacts Kairi as well-as he is the one who sends her Destiny Islands. Xehanort has so much potential...and I would detest to see that squandered on characters who I feel are merely fan favorites and could contribute very little to the plot unless it's forced.
Sora and Riku? Utterly inane-absolutely pointless taking into account they are five year olds playing with wooden swords on a remote island.
Kairi? *snickers* Either I am being repulsively narrow-minded or some of you are deluding yourselves...what the hell can we accomplish with her? Then again...I am looking at this from a combat only standpoint. If it's exploration, platforming, and perhaps some puzzles......it can be done I suppose. However, the only interesting thing about Kairi's past is her departure from Radiant Garden to Destiny Islands (with this only being alluded at with Aqua in Birth By Sleep and further explained in the Ansem Reports.) Other than that, I see no legitimate reason why we should play as her. I should note that I am open to the idea...but that depends solely on its execution.
Ven? Non-sensical for reasons that do not need to explicated on. Unless It involves Ven aimlessly running around in Sora's heart doing god knows WHAT for some currently unexplainable reason If executed correctly...I suppose it could work...but I digress. It would make his ending in BBS far less emotional for me if that were the scenario.
Aqua? Ugh...I do like Aqua, very much earnestly speaking. However...what would her story entail? Wandering with no sense of navigation throughout the realm of darkness? Visiting worlds that have been thus consumed by it? Yes...that sounds wonderful on paper-then you would have to consider the utter lack of interaction in it. It would be prevalently her...slaying a seemingly incessant onslaught of Heartless until she finally reaches Ansem the Wise. We already know how her story is going to end, we were told that much in the end BBS' secret ending. Tacking on anything else comes off as extraneous to me. So...unless she accomplished something momentously plot important while in the Realm of Darkness I see little point in exploring the feasibility further. All in all, though; it's practically confirmed we will be playing as her I...just do not see the reason for it as of now.
Mickey? There's an idea! Playing as Mickey in search of Aqua, Ven, and Terra seems like a nice change of pace. (We all know it's going to end in a cold trail, either way...) I do like the idea of him meeting Xehanort and therefore influencing his search for the "The Door" as well as the mysteries of the Keyblade. Seeing how Mickey goes into the Realm of Darkness searching for Kingdom Key-D is a nice element as well.
All in all...I am completely for Xehanort on this. If this is anything like BBS and there are three scenarios- I pray they would be Xehanort, Mickey, and Aqua. Those are the only three that I can see contributing something significant to the plot.