As per-tradition, the Winter Wonderland event is returning next Tuesday, from December 11th, 2018 to January 2nd, 2019. As with the previous events, we are going to be teased with some festive Holiday-themed cosmetics up until the event starts next Tuesday, with a trailer that showcases the event.
Blizzard World is getting the honours for the seasonal cosmetic makeover, as the map is getting a splash of Christmas cheer (lol no snow in California) as shown in this teaser twitter post:
Like with Halloween Terror 2018, one of the temp. names for the skins was leaked on the Play Overwatch website (but was taken down soon afterwards). There was also a t-shirt design showcasing one of the heroes as a character from The Nutcracker Prince, so like with last year, it's possible that this event will be themed around with a more specific Christmas theme. (That's speculation on my part, though.)
While no new game modes will be added, Mei's Snowball Fight and Mei's Yeti Hunt will return, and some new achievements for both modes have been revealed in the latest PTR update. I'm fine with these modes being fun diversions, and will serve as a nice stress reliever from the daily grind of Competitive Mode.
I'll try to keep this topic updated as I can with new skin reveals and whatnot, so feel free to chip in if I fail on my part. Happy Holidays and Merry Christmas!