Perhaps this may be a story. However, it may be too much for me to write at my current level. So I'll just post a synopsis of each world, or something like that. If I do make a story, or if anyone wishes to use my version as their basis, then...I'll think about it.
The story of this fanfic takes place a few months after the fall of Organization XIII. Without any sign of Heartless or Nobodies, the Keyblade trio Sora, Riku, and Kairi were told to rest at Destiny Island, being that two were gone for so long and one was recently kidnapped. They received many bottled letters, all from their friends from other worlds. It seems all of them were doing well, some getting married. Heck, it was a rather interesting sight when Genie wanted to personally invite them. Other than that, their friends Wakka, Tidus and Selphie simply did not know about other worlds yet, only answered that the bottles were letters from penpals across the waters. Other than that, everything was normal.
World: Destiny Islands
PoI: Main Island, Small Island, Secret Place
Info: A world made of islands. Home to the Paopu Fruit. It seems that these islands hold many secrets and have been visited by other Keyblade wielders.
On another world, things were not normal. Konata was very surprised that the Heartless exists, and even more so when she recognized a white-caped figure, commenting about how this world failed to have what he was looking for. Konata was crazy enough to try to attack the stranger, but without any weapons and without the Keyblade appearing in her hand like she dreamed, she and her friends were sucked into the darkness.
World: Lucky Paradise
PoI: Ryoo High School, Animate Ikebukuro
Info: A peaceful world. Though not ever having contacted other worlds, the shows and games seem to show other worlds.
I might as well cover what four of the Lucky Star crew are doing every since the fall of their world:
Kagami lands in Radiant Garden. She's in shock, both from losing her home and from the characters surrounding her. When she got injured from a heartless and then healed by Aerith, Kagami delved into white magic.
Tsukasa lands in Twilight Town. She is befriended by Hayner's group and later attempts to befriend Seifer's group. After failing some chores, Tsukasa runs away, is saved from a cloaked figure by Vivi and later studies black magic with him.
Nanako, their teacher, lands in traverse Town along with several others. She stumbles upon Merlin's old house, which still has some grumpy furniture left behind. She is saved by a stranger, and in turn, saves him from a Heartless ambush. She later defends herself with a knife.
And as for Konata, she lands in Destiny Islands.