I love this idea Guttersnipe!
I would make the MC someone living in the building. Someone living in the penthouse could be a really popular/famous character from a bedtime story, like Goldilocks or Red Riding Hood. People living lower down could be lesser known characters like... a shoemaker who makes Seven League Boots (very old folklore) or even villains!
Or it could be the other way around, with rich characters on top, like kings and queens, and poor characters on the bottom, like paupers and thieves
Maybe your MC could get in the elevator and bump into the assassin. The assassin could drop something as they rush out of floor x. Your MC picks it up and it's a list of all the building residents. One name is crossed out. A few hours later, it's reported on the news that that character has died by some old weapon from a story (e.g. magic/poison/sword)
This is your inciting incident. So your MC asks their best friend for help and they set off to stop the assassin before he can reach the penthouse, because if he kills that tenant, then he controls the building. Throughout the story, the assassin always seems to be one step ahead of the duo, and this strains their relationship.
One night, the best friend slips up, revealing that they are the assassin, and that the MC is next on their list. Their motive could be:
Their story was never popular
They were a misunderstood character (like Cinderella's sisters)
No one likes their character in the outside world
Their plot is very embarrassing (like the Emperor's New Clothes)
I have so many ideas for the end. They could have an epic fight. If the MC dies, then that could lead to an anti-hero being formed with the best friend, who now kills the penthouse tenant and controls the building. It could lead to a sequel maybe.
Or if the best friend dies, the MC is left rebuilding his home and finding new tenants. He may also keep the assassin a secret. This could lead to the assassin becoming a story character for the next generation, so he's reborn in the building of stories and is out for revenge.
I may have just gone way too deep into something simple. Oh well. This was so much fun. Thanks for the brain game my friend