Adding new features during holiday events, allow players to celebrate it in game!
As time goes on, celebrate every real life holiday in game! Like Christmas, Halloween, Valentine etc. If for example Christmas hits, every mission should be covered in snow, and it must be snowing. Once in a while you even see Santa Claus riding by (easter egg). Also, every unit and building must have a Christmas design. For example every building will have Christmas bells, garlands and other Christmas decoration everywhere.
As for the units, give them something like a Christmas sweater (gained from their mommies!) over their armor. If they kill a unit, let them say Christmas stuff like ''Merry Christmas... except for you!'', ''Looks like Santa has one less to visit now!'', ''No more Christmas gifts for you!'' as examples.
If players DON'T want this, give them the choice to turn the game back to its original. Celeberating those events is kinda unserious, and some players prefer having a more serious playstyle. Or they just want to get out of Christmas after having overcelebrated it in real life too much.
Totally offtopic thing: Will we PLEASE also get a day/night cycle in the next C&C??? Sometimes its kinda annoying to play a game in full daynights while you play in the night
Makes the players in the mood that its Christmas for example
New designs to make, some more unit customation
Look at the first Rising Sun mission in Red Alert 3, the Soviets are celebrating Christmas!
Also World of Warcraft is celebrating every holiday event, its pretty fun to celebrate it there. They allow some new challenges, more funny loot etc.