I know this is pretty far off BUT its a good idea to get this into motion now so 343 has time to make it great. And UberNicks recent video about this topic reminded me how important these events are and how mediocre last years was.
Christmas events (and others such as Halloween, Easter, New Years, Summer, etc.) are always an exciting opportunity for games to feel fresh and in the moment. Creates great memories of specific times in a games lifespan (GTA, COD, Destiny have been doing these events pretty well over the years).
It needs to feel fresh, impactful and get people excited about playing the game.
Maps:
Think back to Halo 5 Xmas event. All the maps had snowfall and snow covering areas of the maps. This was great, and I think this idea can go further. Definitely have snow and snowfall on every map. Also have xmas trees, lights, presents, decorations, etc. added to every map. This helps gameplay reflect the feeling of xmas.
Gamemode:
A unique xmas themed gamemode is important in helping the event feel fresh and unique. I propose a Snowball Fight gamemode (can be available through customs/forge year round too). Have everyone spawn in with a Snowball styled Oddball. Make it throwable instead of a melee weapon and have a snowy aesthetic plasma grenade style explosion. Should be a kill on impact and explode on impact.
Menu Screen:
Might seem insignificant but having a xmas theme for the background of the multiplayer menu would help with the christmassy feeling and make the game feel fresher during the event.
Weapon Customization:
Add some xmas themed weapon charms, have xmas themed weapon coatings, if possible have the weapon wrapped in xmas lights as an attachment. Have xmas themed kill effects (snowman, santa, xmas tree, holly, present, etc.)
Armour Customization: Xmas themed armour effects (snowflakes floating around the head/wrists, xmas lights wrapped around the spartan, etc.), xmas themed armour pieces (snowman helmet, reindeer antlers, etc.), xmas coatings, etc.
May seem like Im putting too much thought into this but if these events are big and exciting people will look forward to them and they will get people talking about and playing the game again. Use these principles with a spooky aesthetic for a Halloween event, easter event, summer event, etc.