Here's a possibly shitty idea I have. Its not a theory or speculation, just an idea based on nothing in particular. In Ultra SM the original 7 UB's won't come to our world. (Only the new ones will)

Instead you have to travel to their individual Ultra Spaces to catch them. All UB's caught in their own space have a new hidden ability. I haven't thought of abilities for the others but I have an idea for Nihilego based on its concept art sheet.

Its art sheet hints that the reason it transformed when it joined with Lusamine is because it consumed Z crystals she had on her.

The ability is called Codependent (Not sure if the name is too edgy for Pokemon, maybe it could be changed. The name doesn't really matter though, the effect does)

The abilities effect is as follows. When Nihilego uses a Z move, it changes into its other form. The form change takes place before the Z move is used (During the energy charging animation) so any base stat increases the form gains are applied to the move.

The form would get an increase in Sp.Attack, Sp.Defence and Defence.