Theory 2: Similar to Theory 1, except Sora travels back in time and realizes that his existance is what caused the mess to happen in the first place so he, in guilt for Ven who probably accepted this a long time ago, erases himself which, as shown in the cutscenes ending of Secret Episode-Birth By Sleep, Sora and Ven both disappear and everyone is still left to handle Xehanort, only without Sora or Ven.
Theory 3: Sora traverses through out the final world to rescue everyone's hearts and restores them to life before they confront Master Xehanort one more time and, both Repliku(Riku's Replica) and Saix help out and the latter sacrifices himself along with Terra and kill Xehanort, they return to the Realm of Light and return to their home worlds where everything is restored and Repliku joins Sora, Riku and Kairi to live in Destiny Islands(via the earlier scene of the two Riku's sitting together at the beach in one of the earlier trailers) and Sora and Kairi share a papou fruit and share their first kiss(their moment together also shown in one of the earlier trailers).
Theory 4: Sora dies after defeating Master Xehanort and rejoins everyone who died before the final ultimate battle as the new Princesses of Light begin to protect the Realm of Light, with someone else taking Kairi's place among the ranks yet Lauriam, Braig and/or the Master of Masters, appear to begin new troubles and the fate of the worlds lies in a new hero, while Minnie and Daisy are grieving for the losses of Mickey, Donald and Goofy.
Theory 5: Terra sacrifice himself while Xion and Roxas fade away, and while Aqua and Ven both leave to find a way to restore and find Terra, Lea and Namine move to Twilight Town while the Original trio and the Disney trio return to their home worlds.
Theory 6: Sora loses and Xehanort rules over and rebuilds the Realm of Light in his image and is thus to remain as the main Antagonist of the Kingdom Hearts Franchise until the end of time.
Theory 7: Similar to Theory 1 and 2. Not long after the defeat of Master Xehanort, Sora, thinking that there is no hope in reviving his friends and restoring their hearts, is confronted by a fully restored Lauriam and Elrena(Or Elenar, depending on how the English version of that scene turns out next year) who now seek Sora's aid when Lauriam/Elrena confess that he/she wants to revive his/her sister Strelitzia and promise to help him revive all of those who died by Xehanort's hands. Sora is skeptical, but seeing how he has no other optains, reluctantly agrees to join the two and they set off on a new journey which leads into a "Revivial/Lauriam Saga" or whatever it would be called in Kingdom Hearts 4 someday.
Another theory: Riku is having a vision of his replica, and that moment with Sora and Kairi is their last moment together before the final battle as everyone probably accepted that their lives are lost in this battle so they return to their homes one final time to have their last moments with everyone they care for, or in Lea's case, to have his last moment in Twilight Town.