i think BBS's secret ending will be something like the keyblade's future.we'll see KH1,CoM,2 really fast,like terra or ven already know what will happen.let me explain...
*THIS IS COMPLETELY HYPOTHETICAL*
Terra defeats MX and saves the world.he knows he is going to pass on his powers to sora or riku and wonders what lies ahead,regarding the keyblade.(we are shown some flashbacks as to what is going to happen in KH1,CoM and KH2)...but terra knows that something else is also going to happen,something the rest of the KH characters are missing (maybe that means the history repeats itself or something)...and that would lead us to KH3!!