A letter from Ther Kuldir has arrived in the arms of General Tiberius, a desperate plea for help from the prince of the locked down city:
To the commander of the Constantinopolitan Forces:
I make this urgent request to ask for aide regarding the state of my city; the Paragon and leader of the military has gone too far.
He has commanded martial law for years now, making the state of garnering aide dire.
My father has been holed up in the palace and unable to leave; I was only able to escape with a small band of soldiers.
I fear he's finding a deeper, darker power. Paragon Atlan must be stopped.
- Prince Orimmul, Heir to the Throne

As far as I know, Ther Kuldir has nothing planned, so I would like to make this suggestion for a questline and boss concerning the city, focused around the power-mad Paragon of the city, Atlan. Atlan's rank as Paragon has allowed him unrestricted access to the military of the city, and has declared martial law. No one enters, no one leaves. No one's heard from the city in ages as a result.

The Paragon's been paranoid of the Frost Queen and finds that the city of Gelayen poses too large a threat to the great city nearby, and wants it destroyed. The player's goal is to uncover just what Paragon Atlan is planning and putting an end to his tyranny. This would consist of stopping the soldiers supporting Atlan, and learning bits and pieces of his scheme to unless a force of nature that lies deep within the mountain city.

The boss itself would be Atlan fused with an entity similar to the Blight, in nature, but with the powers of flame and destruction, known as the Infernal. Ideally, it would be a boss with lots of AoE abilities, and channeling abilities. Many of his attacks would be able to be stopped through attacking him directly or objects he summons, similar to canceling Guillotine from Bandit King. It's an all or nothing battle - end the tyrannical Paragon's reign, or be burnt to a crisp, along with the city of Gelayen.

Credits to ThePanda_14 for helping out with lore pieces. Any questions I can try to help answer! I left a lot vague for dev and staff interpretation!