For a story line you could have the pre-req. of the Dark Brotherhood for sure and maybe the Thieves guild. A group of pirates that escaped after Fortunata's Fall starting a new life on a small island. This particular class could have some passive abilities in on of the tree's that utilizes party mechanics. Gaining unique buffs from the number of players that are teamed up with that would modestly boost the active abilities performance. Another tree could focus more for soloing that has debuff attributes with dual wielding like dropping the atk, def, stamina, magicka of an enemy by a percentage. And another that could do something else unique for its class.
These are just general ideas, creative suggestion that can be inspirational of what the class can do. Obviously not to make it more powerful than any other but allow for more entertainment during gameplay.