I'd like to a make a theory.
When MF has showed that No Name keyblade is indeed a clock, and a hourglass heavily supports time travel. 3D will be when the Mf goes back in time to KH1 that he will use future villains (Dusks, Org., etc.), all against Sora, and Riku. Meanwhile the falling Sora's could represent all the other Sora's of the other timelines (Riku leaves a hole in this though), while Sora, and/or Riku can defeat MF gain the No Name keyblade, and make it possible to really save Terra, Ven, Aqua, Xion, and Roxas.
The idea has holes, but I'd like to think it's make some sense.