While you're at it, don't hold it against me for taking a month to respond. I've been lazy about KH lately, I'm waiting to see spams of hate mail from my fanfiction fans for not finishing Falling for a Nobody yet. XD
Nothing wrong with second guessing yourself in your writing. It would probably only help you improve the story.
I HATED, absoultely DESPISED, that they brought in TIME TRAVEL! It's so unnecessary! The game could have progressed fine if Xehanort had returned as Terranort and had a different plan for resuming the Keyblade War.
Chances are, whatever the next saga is, it won't live up to this one. Sequels are generally hard to make, and considering what the series has done so far, it's gonna be tough to surpass it. But money talks, and the games are still very successful, so I'm expecting to see new KH games a decade from now even XD.
I've never been very accurate with predicting what Nomura will do, but one possibility I've considered is that the new vessel of darkness is Ansem the Wise.
If you've noticed, there's something strange about Ansem and Xehanort's Heartless and Nobody. The 3 of them all have ORANGE eyes. Not yellow, orange. Now there are two possibilities:
1. Ansem the Wise always had orange eyes, and his experiments on Xehanort eventually caused his eye color to change.
2. Orange eyes signify another level of Xehanortification. A weaker level, that is, since Xemnas and Ansem SOD seemed to care about their own plans more than MX's backup.
Example: Xemnas says how he will "devour all... the light and the darkness" during his battle against Sora and Riku.
Now, we know what color Apprentice Xehanort's eyes are. They're brown. In both BBS, and the KH2 flashback, they're brown.
Now, it's possible, though unlikely, that Ansem the Wise performed a final experiment on Xehanort after the flashback in KH2 with Mickey. This experiment may have unleashed MX's memories/ influence and caused the eye color change.
However, I think that's less likely, and I feel that the biggest spoiler in the saga coming up...
...is that Xehanort WANTED Ansem to insert his data within Sora. Perhaps as a way to give the Lights a false sense of security and fight the 13 darknesses at once. Or perhaps the data contains a secret vital to using Kingdom Hearts' full power, something that Xehanort never understood or possessed.