So, 7 guardians of light to protect the seven princesses against the 13 Darkness.
Now, for simplicity, that would mean each of the good guy wielders would get to protect one of the princesses against one or two members of the 13 Darkness. Now, doing it that way gives each wilder a chance to be playable for some time and more exposure for those Disney worlds that the princess belongs to (sorry folks, but Agrabah is coming back, hopefully using the TV series as the plot for the world).
However, Kairi is the problem here, being in both groups. She can't very well protect herself since that would defeat the purpose of a Light Guardian protecting a princess. Also due to the fact that out of the 7 wielders we assume, she has the least battle experience. So odds are that she is paired with another experienced wielder, some body guards (she teams up with Donald or Goofy?) or is placed in a secure area to train or to be protected, or someone we don't expect appears to fight with her (Xion.....?).
With the math set-up, there are 7 guardians and 13 darkness, so you would assume that almost every guardian would have to fight or deal with two of the darknesses, leaving one guardian against one darkness. Odds are that could be Sora against Master Xehanort in the final battle.
It appears that the plot of KHIII will involve protecting the princesses, perhaps failing so the X-Blade could be created (failing by perhaps lea being control by MX, Vanitas popping up or if Terra is saved he is taken under control again or something, basically where one or more of the Guardians is manipulated by MX to do his dirty work). So, i was thinking that BBS is where someone like Aqua or Ven will be rescued and BBS2's plot is getting the group set-up for the big fights in KHIII.
I wonder if we will see the Keyblade of People's hearts from KHI again as that was created from the hearts of 6 of the princesses. It makes it appear that that was a proto-type X-Blade if this is what happened to the X-Blade after the war.