That really depends on the community, although I'm not in the wiki-forums that often, they tend to be some magnitude more serious than other Forums I know.
As for who kicks Xehanort's ass, I can think of 3 possible ways instantly:
1. Sora finally finishes him off in an one vs one.
2. Everyone gets a shot at him and Sora only delivers the final blow.
3. Somehow Xehanort manages to destroy himself, lol.
Honestly, Sora going bad/becoming evil would be the oldest "twist" there is and it would also not tie in with the claim that "The KH-series remains Sora's story even if Xehanort's is done.
About the BBS FM-secret ending, I think the whole cutscenes are the secret ending, yet still I haven't watched a gameplay-video of Secret Episode yet so I can't say what it's opening is.
On to KH 3D: DDD.
To start one speculating on its contents we must recall what mysteries there still are and what we already know for facts.
1. Fact: We know from Nomura that the final battle with Xehanort and the rescue of the suffering ones will take place in KH III.
2. Fact: The Re:Coded Secret Ending hints at Sora's and Riku's Mark of Mastery-Exam being held by Yen Sid (and possibly Mickey, too)
3. Fact: The secret ending furthermore tells us that Mickey and Yen Sid were actively looking for Terra, Ven and Aqua. In their conversation Mickey mentions that he has an idea where Ven's heart might be, to which Yen Sid nods in approval and then remarks only Terra remains.
4. Fact: Due to the above conversation we can deduce that Mickey and Yen Sid know that Aqua is located in the RoD.
On to mysteries:
1. The data Ansem hid inside Sora, what is the content of it? How is it supposed to be the key for rescuing the tormented ones? How can it be accessed?
=> Exploring this mystery seems a good candidate for me in 3D, because knowing about this may be a prerequisite for KH III.
2. Ansem, Seeker of Darkness and Xemnas, Master Xehanort's heart and memories, Eraqus's heart and memories. Terra's heart and memories. How all this big mess was devided and who had what, as well as what is with them NOW (at the time of 3D) has a high chance of getting light shed on.
3. Where is Terra? Or what remains of Terra? Learning about this could also be a prerequisite for KH III, as an attempt to rescue Terra can only be made if they know if it's possible and how.
Furthermore, we still don't know what kind of torment Naminé and Xion actually have, so a slight chance of this getting a spot in 3D coverage is also there.
Then there's the overall theme of "trust". As said in Blank Points, Sora and Riku are seemingly those who are destined to save the universe. For them to become a real unstoppable force they may have to learn to trust in each other in every possible way so that they can't get pitted against each other again like in KH 1.
I see, that's of course a hindrance too...but maybe you should look up some characters on the FF wiki. I actually played FF VII, VIII, IX, X, X-2, XII and XIII (still in progress) for myself, but I also sort of "know" the characters from FF I to VI.
In Dissidia I can use every character except Jecht (he has super strong Combos, but I suck at getting the timing right, which is crucial for Jecht), but I mainly use either Golbez or Sephiroth.