Who's ready for a crackpot theory?
I think Scala Ad Caelum and Destiny Islands might all be on the same world. Hear me out.
Both are worlds important to Keyblade Wielder history, Destiny Islands being where Xehanort was raised, where he took Ventus after stripping him of the darkness of his heart, and where Sora, Riku, and Kairi began their journeys, as well as where everyone goes at the end of KHIII.
As well Scala is the homeworld of many a keyblade wielders of the old generations, though Eraqus and Xehanort are the last ones known to still be alive at the time. Luxu is also, sort of, from this world. Though in his time it was known as "Daybreak Town". Xehanort also chooses to create a copy of this world for the final battle of KHIII.
As well, both Destiny Islands and Scala are Island areas. It's possible they share the same sea.
We also know all worlds were once one world, and were rebuilt from pieces of light in the hearts of children, as said by Kairi's grandmother, said to be Ava now grown old with time. If she is Ava. She would know
I call this theory the One Sea Theory