One possible fanfic could be about Brynjolf meeting the Dragonborn who is wearing the Dark Brotherhood Armor. Maybe the Dragonborn is on a secret mission related to the Dark Brotherhood, and Brynjolf gets caught up in it. They could form an unlikely alliance to face the challenges that come their way, with the unique powers of the Dragonborn and Brynjolf's skills in thievery and combat complementing each other.
The fanfic might explore the backstory of how the Dragonborn acquired the Dark Brotherhood Armor. Brynjolf could be intrigued by the armor's origin and the tales of the Dark Brotherhood. The Dragonborn might share their experiences within the Dark Brotherhood, leading to Brynjolf having mixed feelings about the assassin's guild. It could also involve a quest where they have to use the armor's special properties to overcome some ancient evil.