Within Pokemon Go we have seen Pikachus in Santa hats as well as more recently, party hats. To go along with this, there was an increased general spawn rate of Pikachu there were variations to make the events more festive. In general, these events have coincided with holidays/observances, but there are just a few more that Niantic should make available.
Squirtle Squad
As mentioned previously, there have been events where Pikachu has been wearing Santa hats as well as party hats. So a slight variation on this, could feature an increased spawn rate of Squirtles and even giving them the robes and sunglasses that are associated with the Squirtle Squad from the anime.
Increased Magikarp spawning/Red Gyarados
Depending on where you might be, Magikarp can be a little difficult to find. One event could increase the spawn rates of areas that aren't hugely populated. In addition to this, there could be a substantial chance of obtaining a Red Gyarados once the player has the 400 Magikarp Candies required.
Increased Spawning of Dragon types
I must have the worst luck of all players, but I am one of the only players I know that does not yet have a Dragonite. So there could potentially be an event wherein spawn rates of (at the least) Dratini to potentially spur players getting Dragonites, etc. In my personal experience, I have not run into a great deal of Dratini/Dragonair and walking 5K to get 1 Dratini Candy is ridiculous.
Using one day to distribute "region exclusive" Pokemon
Region Exclusives Gen 1
- North America: Tauros
- Japan: Farfetch'd
- Australia: Kangaskahn
- Europe: Mr. Mime
Region exclusives Gen 2
- South America: Heracross
What would essentially happen with this event is giving players all around the world one day (maybe more?) to catch Pokemon within their region that aren't typically available. A player in Japan could try to catch Tauros or a player in America could try to catch Heracross and so on and so forth.