So, Kayn is a new champion so that means he will not be getting new skins anytime soon, however, this is too good of an idea to pass up. It's a match made in heaven/shadows. Of course his scythe/Rhaast would be the candy cane, in Rhaast form his horns could be reindeer antlers and in Kayn form he could be a Christmas elf. Instead of his long hair in Kayn form he could have a long elf hat.
Making Rhaast a full reindeer would be great too, a red nose and everything. And Phreak if you're out there, this could be a great opportunity to get back out into the world of puns, you've fallen off the radar in recent times.
Anyway, those are my thoughts on a Christmas skin on Kayn. This skin will not come any time soon because Kayn is new champion but it could be for next Christmas.