A really engaging plot could be where Shikamaru's shadow blade is stolen by a rogue ninja. Shikamaru then has to rely on his wits and his other ninja skills, like his shadow jutsu without the blade, to track down the thief and get his weapon back. He may also uncover a conspiracy along the way that threatens the entire ninja world.
Perhaps in the fanfiction, Shikamaru discovers a hidden ability of the shadow blade that allows him to travel through shadows to different dimensions. This could lead to him exploring new lands and encountering strange creatures while trying to find a way back home.