I remember in the days of Halo3 Bungie really knew how to celebrate in holiday style by adding little treats in the games like floating hearts on Valentines day for example. I think it might be a cool idea to have guardians explode into a show of fireworks if they are killed by a grenade on the 4th. Or just add a consumable like the Halloween Jack'o lanterns where if you die or throw a grenade they go up in holiday style. Or maybe just get a shader/class item that glows in red white and blue or something....