Snow can pile up already in MC, but having it pile up in colder climates during the winter would be a nice immersive feature.

Probably not gonna happen. Having it snow in non-snowy biomes in winter can already be pretty intensive on lower-end hardware (Lots of chunk updates happening at once), and this would most likely only add to it.

We'd likely need some kind of "fancy" algorithm to make it not look ugly either (Cause just doing it randomly like snow already is would probably look really bad), and that would just add to how resource intensive it is.

Hm, Winter Wonderland managed to do it, maybe some support for it? Or at least, see how they did it? It has various config options and what not to make it less ugly.

https://minecraft.curseforge.com/projects/winter-wonder-land