K/DA... I think I'll need to see the particulars.
This came up a little while back, but they need more cards from somewhere. They're making new champ cards faster than Riot is making new champs, after all, but they're not making them faster than they're making new skins. So just from a content production standpoint, it makes more sense to make skins into champ cards rather than into alt art for champs.
Furthermore, a lot of skinlines will seriously change a champion's themes, so it makes sense to give that some mechanical recognition. Sometimes those shifts are cohesive enough between champs, and the universe the skins pull from are big enough, I almost think you could make them a region. Pulsefire, Project, Odyssey, and Program are big obvious ones. MTG has the existing planewalker lore to justify why all these different settings are in one game, they could do something with that here, or just handwave it. But would I love a High Noon set? Hell yes. So whatever that takes.
K/DA... I don't think you could do a K/DA region, lol. But maybe a musician-verse if they were willing to get a little comical with it. K/DA, True Damage, and Pentakill is actually a pretty solid roster. But again, it'd have to be a comedy region, because what do you do with the followers other than groupies and roadies and shit, so maybe not.
For these, it'll be weird for now. But they could establish a precedence here for event cards. Breaking new ground, it's kinda exciting.