"2. The King of the Ducks
A secret door that is unlocked after collecting all the Anomalous Ducks. After collecting all the Ducks you have to do the following steps:
(I leave this part to the developer's choice)
Inside the room is a giant rubber duck with a crown on it."
Perhaps that secret door could be SCP-249. I imagine, upon following the requirements by collecting the duckies and finally entering through 249, you'd be transported to a seemingly non-existent location comprising thousands of animated duckies of different variations. Stereotypical joyful music is being played in the background, and the surroundings is full of vibrant colours. 20 seconds in, the whole surroundings gradually fades away, to the point where all duckies disappear and the music dies abruptly. The player realise that their character has been hallucinating, and find themselves in an abandoned toy workshop with few non-anomalous duckies scattered on the floor.
At this interval, the player could either exit through 249 or freely roam the abandoned establishment for further clues. If the player choose to roam the building, they may encounter a ritual attributing to the duck toys, and perhaps along with a note relating to its background history.
IMO, I'd love if the duckies have a somewhat iconic and humorous figuration within the game, or in other words becoming a "celeb" to SCP Unity. I would love if the duckies are a recursive subject throughout the game... like taking similar notion to the companion cube from Portal, something like that? Absurd as it sounds, if the duckies actually had some significance to the player character's background (if executed rightly), I'd ship it.