A possible fanfiction plot is that Grell and William are given a mission to protect a special soul from being corrupted by a powerful demon. Along the way, Grell's flamboyant nature clashes with William's strict sense of duty, but they find that their combined skills are the only way to succeed. Or perhaps there's a plot where they get trapped in a magical realm and have to find their way out while dealing with their own complex relationship and the strange creatures in the realm.
Some fanfictions might explore a plot where Grell accidentally gets a part of William's memories. This leads to Grell seeing a different side of William and trying to get closer to him in a more understanding way. There could also be a plot where they are in a competition for a promotion in the reaper world. However, they end up realizing that they are better off working together than against each other and form an unlikely alliance.
One interesting plot could be Grell and Undertaker teaming up to solve a mystery in the underworld of the Black Butler universe. Maybe they discover a new kind of demon or a secret about the reapers that has been hidden for ages.
There are many 'Black Butler Grell fanfiction' that focus on his unique personality. For instance, a fanfiction might show Grell trying to improve his reaping skills in a more humorous way. He could be constantly getting into trouble while attempting to be a better reaper, like accidentally scaring away the souls he's supposed to reap. Or, a story could be set in an alternate universe where Grell is not a reaper but a normal human with his flamboyant personality intact, and how he navigates the human world.
One interesting fanfiction could be about Sebastian and Grell being forced to work together on a special mission in the human world. They start off hating each other but gradually develop an understanding. Another might involve Grell having a crush on Sebastian in a more comical way, like constantly trying to impress him with her 'reaping' skills but always messing up. There could also be a fanfic where they are both trapped in a magical realm and have to rely on each other to escape.
One interesting storyline could be a time - traveling adventure where Grell and Sebastian are forced to work together to fix a disruption in the timeline that threatens the existence of their world. They might start off hating each other but gradually develop an understanding.
Grell is impulsive and often acts on his desires, while William is more calculated. In fanfiction, this can result in interesting dynamics. Grell might drag William into unexpected situations, like chasing after a rare soul just because it looked interesting. William, on the other hand, would be trying to keep everything in line. Their personalities also have room for growth in fanfiction. Maybe Grell learns to be more disciplined from William, and William learns to loosen up a bit from Grell.
The theme of redemption is also quite common. Since Grell has done some rather dark things in the 'Black Butler' series, many fanfictions explore whether he can find redemption. Some stories show him trying to make amends for his past actions, either through helping others or by changing his ways. And of course, the theme of friendship or camaraderie among the reapers, especially with Grell, is often explored. We see how he interacts with other reapers and how those relationships develop or deteriorate in different fanfictions.
Another theme is 'Redemption'. Maybe Undertaker is trying to redeem Grell from some of his more... extreme actions. This could lead to a lot of character development and interesting plotlines.
Another plot could be a love triangle between Sebastian, Matilda, and Ciel. Matilda might develop feelings for Sebastian, but Sebastian's loyalty is first and foremost to Ciel. This could create a lot of tension and drama as Matilda tries to win Sebastian over while he is torn between his duty and any possible feelings he may have for her. In some fanfictions, this might lead to Matilda making some desperate moves that have far - reaching consequences for all of them.
There's 'Grell's Hidden Depths'. This fanfiction focuses on Grell's inner thoughts and feelings. It shows a side of him that we don't often see in the original series. It has a lot of emotional moments and some really creative plot twists that make it a great read for any 'Black Butler' and Grell fan.
One common theme is family secrets. Since Grell already has a rather mysterious character, adding a sister just multiplies the secrets. Another theme is identity. The sister often has to figure out who she is in relation to Grell and the other characters in the Black Butler world.