I reckon UBs should be treated kinda like a mix of Team Plasma Pokemon and Ancient Trait Pokemon.
So they'd be like Plasma Pokemon in the fact that they would be distinguished as Ultra Beasts like the Plasma Pokemon are distinguished as Plasma Pokemon and that comes with supporters, items, attacks, etc. specifically effecting them. Eg a Lusamine supporter card that lets you search your deck for an Ultra Beast, put it onto the bench and attach 2 basic energy to it or something like that.
Then the ancient trait part would be their "Beast Boost". So it's at the top of the card like an Ancient Trait is but instead it says Beast Boost, and each Beast Boost would be an effect related to the amount of Prize Cards you've taken eg 10 more damage for each, 1 less (C) in retreat for each, damage done to this Pokemon do 10 less damage for each, etc.