First of all, I would like to thank both my good friends Revalationrevo and Com-Zerash for their great help and inspiration.
You, the commenters, are always welcome to suggest changes as well as show support.
Shall we begin?

So, every day (about 5 days) before the actual birth date of SK, a distinct and exciting prestige mission will be offered from the prestige mission menu. The premise is the same every day: Biscotti is asking you and your squad to invade either a gremlin chef outpost throughout Cradle or visit a Spiral Order supply depot taken over by gremlins (though they may be hidden and reveal themselves in deadly traps). In this way, Biscotti will have the help she needs to get the birthday bash’s cakes ready in time without resorting to dark matter to make them faster. Each day’s prize will be some cake tokens and the mission’s dropped loot (rarities, spark tokens, crowns, materials).

On the day of the Bash, Biscotti will enlist as many knights as she can to deliver giant cake slices to those of the Order living in Haven. Your squad will be enlarged to a capacity of 6 (maybe more?) and each member of the squad will spawn in a different warehouse that is part of the Bazaar. You will pick up a cake from the stack of them, and a cut scene will show you saying to yourself “Just a taste.. Have to make sure Biscotti did it right this year *laughs*.”
All of a sudden, either a knight chef, an Emberlight chef, or Groat will come into the room and throw a frying pan at your finger. “What’s the big idea?” then the pan bounces off the wall and hits a cake, setting off a substantial explosion. When you look down, you see the frosting is knocked from your finger. You:“Nice shot! What’s with the cakes?” Chef: “Gremlins sabotaged the cake frosting, making it explosive. The Gremlin chefs have baked a huge batch of creep cakes, current cakes, and explosive cakes! Everyone is relaxing because of the Birthday Bash, it’s the perfect time for a surprise attack! And now they have an army!”
You:“We have to tell everyone!” [Try to send message to Kora, but the network goes down almost immediately after massive explosion with burning frosting raining from the sky.]

Exit the building into the bazaar and shout to all of the people saying that the cake is explosive. Joined by NPC chef knights (including Groat) and your squad of up to 6 knights. You proceed to the arcade where the place is infested with hordes of creep cakes, boom cakes (explosive cakes and their robot variants) and current cakes as well as gremlins. Explosive frosting blocks will end up all over the place. Gremlins of all types are there as well as pushing carts that have battlepods and gun puppies on them. They don’t work until they are placed down (the puppy carts are fast, the pod ones aren’t), and they are vulnerable until then, but well defended and lightly shielded nonetheless. Menders cannot heal anything on the carts. The battleground includes ground past the entrance to Haven, town square and arcade. You don’t have to load between those sections of Haven for that battle. Objectives: protect Strangers and NPCs, clear battleground. Bazaar>Arcade>Garrison>Town Square/entrance. The snipes will help in the background by slicing the cakes with their spinning wings. While in the Garrison, Feron tells you that the Lab and Hall of Heroes must be liberated from Gremlin siege. Clear out those buildings (which all have frosting in drifts on the ground and walls) and the Guild Hall, then fight several cakes in the HoH. When that battle is over and Barrus is rescued, he tells you that gremlins are pouring into the Town Square. In fact, a huge cake is being wheeled in...

When you go to the Town Square, you must fight King Cake who has taken his place on top of the fountain. He will not move much.
Throughout your fight with it, a Gremlin chef will taunt you.
Appearance: Gigantic creep-cake looking cupcake that has candles in the shape of a crown. It has no mouth and will not move much. It can groundpound jump though.
Attacks: Candles from the floor, except extremely accurate, more of them, and more damage. Spins like JK to set things on fire. Can shock and has shocking lasers as well. Will release blast cube explosive gel orbs en masse.
Attributes: Takes more damage when on fire, though the fire does little to it. Immune to freeze, shock, curse. Can be affected (minorly) by stun, poison, fire. Huge amount of health and defense. Spawns all cakes and batterbots, occasionally Dread Velvet. The Dread Velvet will try to take over the King and you must stop it or it will become significantly more powerful.
Supported by gremlins of all types as well.

There are two ways of defeating the boss.
1.) Hack and slash to stay alive. The mobs will drop vials, health, crowns, and heat.
2.) Protect Ozlo and Biscotti. They have limited health (which can be replenished like Impostoclaus) and will spend most of their time charging/reloading special weapons they brought along. They will devastate the King if they are protected. Drop rubble in front of them (which will be knocked all over by groundpounds) to protect them. Biscotti’s weapon in phase one a sword that makes an energy slash that does an random damage type. Phase two that malfunctions and It reverts to Gun mode, which is catalyzer/pulser to the extreme. Then in phases 3 it over loads and Biscotti throws it and it explodes, ending phase 3. Then she uses a Heavy laser cannon that sweeps away the mobs and temporarily lowers the defence of King Cake allowing you to finish it quickly, if you are fast. Then she uses a bigger Mighty great cleaver and a uber power mitt. And Ozlo has an Iron Slug musketoon that can fire 5 charged shots at once in all phases, though only when charged up.
Note: You will want at least one support bomber in your party. Ash of Agni, Voltaic Tempest, Stagger Storm, Dark Retribution venom veiler will all be helpful.

Once it takes enough damage, a cutscene shows someone saying “it’s gonna blow!” and you have to run as the King Cake explodes into many explosive pieces. (as you collect tokens and rewards) and blows up, covering the entirety of Haven in frosting, confetti, candles, and artillery (wait what?!).

Then you fight the horde outside Haven on the city walls defending the entrance to Haven, which is larger because of King Cake's entrance (with the NPC’s possibly) for a little while until the battle is “over”. Kora may call to congradulate your team for a job well done protecting Haven after she gets the comm system back up.

In the days after, prestige missions have you defending Haven from attack. The Town Square will be fortified, frosted and in the background of some places, Cakes will be shown being attacked by knights and snipes. While in Haven, you can share cake with others as a fun little thing.

Rewards: Prestige like a danger mission. Up to two *Cake Crown fragment tokens/materials.
Loads of crowns from the mission itself because everything will drop crowns and heat.

*Cake Crown fragments will be used to craft the Overcharged Mixmaster from the Kilawatt Pulsar or the Power Mitt from the Mighty Defender (or recolors/reskins of those items). This brings those two controversial items into the game for free. Yay!

The days after the Birthday Bash mission (only completable once per year unless that seems harsh) will have prestige missions that deal with supply delivery and/or protection of Haven from the remnants of the cake army. A scene later on will involve a relatively powerful and annoying Gremlin chef being defeated (similar to a buffed Gremlin demo for much annoyance). His remains include a Crimson Order badge, and a note reading: Hah! If you had only funded my project, Haven would be in ruins and the metal invaders gone for good! Be sure to eat that delicious cake that I sent all of you.
-[I can’t come up with good names. Whatever name someone comes up with will go here.]

So, what do you guys think? Like I said, please leave further suggestions as well as your approval. This could become an amazing event for the next year.
~Elt, Revalationrevo, Com-Zerash <<(Much credit to them)