I used to be a WoW player back in the day and was a very dedicated player. Even though I loved raids and leveling alts, one of the many things that kept me coming back was the random and holiday events they routinely brought out. For the week of Halloween they would put decorations on buildings and have special public events show up on occasion or a holiday twist on dungeons n such. Then for the week of Oktoberfest they would have events where they would have mini-games and special collection quests that could be turned in for holiday currency that would allow you to buy silly cosmetic things only available for that time (pumpkin heads, auras, etc).
I think this would be great to keep casual players coming back and something for hardcore players changeup from their routine tasks. The rewards/gifts from the holidays don't have to be advantageous in any way, could just be cosmetic or even temporary. I would like to see these holiday or even made up events every couple of months just to keep everything fresh. They could be entirely made up or twists on actual holidays but given different names so no one feels excluded.
As for the rewards I would like to see maybe design shapes for the javelins, maybe something as far as holiday/unique takes on guns (like ornaments on Destiny), to even different auras or thruster designs/colors. I just think it would be a fun way to keep players interested and coming back constantly even in the later years of the game