I was thinking about what I could do as Ninantic to capture as much of the Xmas Money as I could.
Xmas = Lots of Google Play / Apple Gift Cards = huge temptaton to put limited time things to tempt Pokemon players to spend those Gift cards on Pokemon Go.
My money grab idea was to introduce Gold Eggs into the Marketplace.
Gold Eggs: 20km eggs for sale in Marketplace for 500 pokecoins + tiny chance off a stop. Only available for December 20th to Jan 3 (2 weeks)
Gold Egg Hatch List:
Mew, Mew2, 3 Legendary Birds, 3 Region Specifics NOT from your region, would give you 8 possible.
The 20k distance + 2 week window of buying these eggs (or finding one), would dramitically limit the numbers and candy available for these eggs. But if you wanted to further reduce the impact on the game you could also add in a few non-legendary fully evolved pokemon to the Gold Egg list. Like 3 final forms of 3 Starters (Charizard, Venussaur, Blastoise) or things like Dragonite.
PS: I called them gold eggs, because it would be Pokemon digging for Xmas Gold :>