Only one I could possibly see is Vor, and that's because the timeline fits. Xehanort was an old man in BBS and Kairi's grandmother is obviously up there in age herself. There's also the obvious elephant in the room being, how in the hell does she even KNOW about something like this??
I mean, yeah, there's now an instance of blood-related connections in Kingdom Hearts, so the precedence IS there. I don't think everyone here in Dark Road is related to everyone currently in the forefront though.
Now, Radiant Garden = Scala ad Caelum? Hm. On one hand, it's a bit of a stretch because geographically speaking, they don't look alike whatsoever. Although... Scala didn't look like Daybreak Town either, but it's built on top of the ruins all the same, so who knows. And Castle Oblivion didn't look like the Land of Departure at all, despite LoD being the original form of that world.
Two, Scala seemed to be a world that existed at least up to 75 years ago (maybe even closer to Present Day than we think). If that were the case, then people like Ansem the Wise and Kairi's grandma would absolutely know about the change, since they'd be alive closer to when it'd occur. Not to mention, there would have to be records of the change. Maybe Tron would have data on that too, and if Riku is currently in Radiant Garden... This might be a perfect opportunity for something like that to be explained, especially as a Keyblade Master himself.
And three, we don't actually know what happened to Scala either. For all we know, it could be in existence perfectly fine but just difficult to get to for the sake of protection. Land of Departure turned into Castle Oblivion, and LoD is a place that definitely took inspiration from Scala, so I'm sure Scala itself has many preventative measures that exist should the worst happen. Especially since LoD transformed into CO with knowledge that Eraqus passed on to Aqua, so it's something only Keyblade Masters should know. Since Eraqus and Xehanort studied in Scala, I'd have to think Eraqus learned it from there.
On the other hand, it's also not like Radiant Garden hasn't had something similar happen to it before. I mean two years ago (as of ReMind), the place was still called Hollow Bastion because of Maleficent taking it over 9 years prior and destroying it. As well, Radiant Garden may not look like Scala ad Caelum, but it does share similarities with Daybreak Town. And if Radiant Garden = Scala ad Caelum and Vor = Kairi's grandma, then it'd make sense because Scala is supposed to be the hub for all Keyblade wielders, and we see Kairi's grandma in Radiant Garden. Daybreak Town was destroyed by something and became Scala, but if something did happen to Scala and Keyblade wielders were present during this (and they would be), I'm sure they'd act in some way to save their home.
But to pull a Nomura here... What grows in a garden? Flowers. What flowers hold the most significance in the story? Dandelions. What do you call light that glows brightly? Radiant. Personally, I think when the World split, parts of Daybreak Town did too, and that's why we have Twilight Town and Traverse Town also in existence. They all form a different purpose, but they're originally part of one bigger world. Not to mention, the buildings in each town do look sorta similar. You never know.