>>32472392Dunno, I think Primarina doesn't really need a Mega to become OU.
But if we have to go for it, the Ability could be Water Bubble and the stats point could go in Primarina's defensive stats.
If Water Bubble is too much, than just a powered up version of Liquid Voice would be ok too. Primarina has the typing and moves to be good already and even just the natural boost to the BST of Megas would be enough to make it broken, with or without good abilities.
If we are actually getting new moves or abilities in the future, I think Z-moves are more likely though, but I'm not sure if one could change that much. Maybe something powerful like Oceanic Operetta that also makes it rain after it is used?