Always cool to have some sort of holiday event. adds a little bit of needed silly escapades in times of boredom and what-not. Cryptarch could dress up as "Scrooge" and offer special deals for an exorbitant amount of coins for Christmas. Also dress the robots in elves costumes around the tower complete with pointy hats. Why? for shits and giggles. And have the faction whom you have helped the most send you a Christmas package in the mail with a purple engram. Halloween? Maybe have a "trick or treat" trek through the hive on the moon where sealed off doors open when you interact with them: An Acoylite peeks out and drops a random engram on the ground or drops a grenade instead followed by evil laughter. that's all that comes to mind currently, but will update with more ideas