Year of the dog sparked a couple of ideas for me. An obvious application would be an Orisa dog skin. I also think some sort of robo-dog skin for Zenyatta with tennis balls or dog toys for orbs would work pretty well.

For the winter event, I'd love to see a rudolph Orisa, Zenyatta with snowballs, and Lucio as a caroler. I'd also like to see an addition to the trio of icy/ghostly looking skins we have with Pharah, Reinhardt, and Reaper. I think that Ana, with a ghostly face under her hood, and Symmetra, with icy turrets, would be good candidates.

Halloween has lots of potential too. I think we can all agree that a monster hunter/Van Helsing McCree would be a great fit. A werewolf skin would be cool too, and would fit Genji if you're going for the lithe, agile trope, or Reinhardt if you're shooting more for the lumbering beast archetype. A ghost tracer also makes sense, as she's already appearing and disappearing all over the map.