I am very very relieved that Carbink has been confirmed to have some relation to Diancie through this reveal. The similarities were too similar for their not to be a connection.
As for other possible connections I briefly listed, I will explain a bit.
I listed Avalugg for Volcanion simply because it seems most likely, if I had to choose a pokemon that underwent a mutation or shared some common ancestry. They have similar body shapes and the divergence of ice to steam body doesn't seem like too far of a stretch if extreme environment changes took place transforming H20 from a solid to gaseous state. But again, I don't 100% or 50% think this is a connection, it just is possible that the other Legendaries also underwent some freak mutation at the same time from some sort of event.
Hoopa, the closest this generation would be klefki, but, again, it's a stretch. I just went with that klefki since they have vaguely similar aesthetics (klefki is a hoop without its keys, thus "hoop"a the"Mischief Pokemon...that gathers things it likes and pushes them through its loop to teleport them to a secret place." Perhaps a klefki unlocked an ability to not just use it's ring/hoop for storing it's valuables, and instead harnessed an ability to teleport valuables. Also, the prankster/magician abilities could relate to the psychic hoopa abilities. Perhaps in death/ascension a klefki's abilities transformed. The type change would be the most difficult part to explain, but it is possible.
I like these three legendaries, and they all seem connected to energy, thus, it's fun to speculate if all three underwent "energy" transformations. High pressure carbon (Diance), high temperature variation (Volcanion), death (Hoopa).
