Another plotline could be a time - travel story. Twilight accidentally travels back in time to when Sombra was first becoming evil. She tries to change his path and prevent him from turning into the dark villain he became. This would involve her getting to know the younger Sombra and trying to influence his decisions with her knowledge of the future.
A common and interesting plotline is Sombra's redemption. Twilight often plays a key role in this, like in 'Sombra's Redemption' where she helps him see the error of his ways. Another is their teamwork. In 'Twilight and Sombra: Against the Dark Forces', they team up to fight a greater evil that neither could face alone. It shows how their different powers can complement each other.
A plotline could involve Sombra's spirit lingering after his defeat. Luna, being attuned to the more mysterious aspects of Equestria, detects this. She tries to communicate with his spirit to learn more about his motives. This could lead to a journey of self - discovery for both of them as they explore Sombra's past and Luna's role in understanding the darkness within Equestria.
One aspect to consider in this fanfiction could be the character development. Twilight is often portrayed as a studious and kind - hearted character. In a story with Sombra, her intelligence might be used to outwit Sombra's more devious plans. For example, if Sombra tries to take over Equestria again, Twilight could use her magic knowledge to counter his dark spells.
One common theme is power struggle. King Sombra is a powerful and often menacing character, and Twilight is also very powerful in her own right. So, their interactions can be centered around who has more power or how they can use their powers against each other.
There are indeed some interesting ones. However, the quality can vary greatly. Some might focus on a more antagonistic relationship between King Sombra and Twilight, where Twilight has to use her magic and wits to overcome the threats posed by Sombra. Others could take a more unexpected turn and show a possible alliance or even a romantic relationship in a non - traditional sense.
A plotline could be that the protagonist is a new pony in Ponyville who catches Twilight's eye. They start to interact more and more, and a romance blossoms. For instance, they meet at the library when the new pony is looking for a rare book.
One possible plotline could be Luna seeing Twilight's potential and taking her under her wing to teach her some ancient and forgotten magic. This would lead to Twilight having unique abilities and a special bond with Luna.
One great fanfiction is 'Sombra's Redemption'. It explores Sombra's character development in relation to Twilight. Twilight tries to understand Sombra's past and help him find a new path. Another is 'Twilight and Sombra: An Unlikely Alliance' which focuses on their coming together to face a common threat in Equestria. It's full of action and adventure.
One possible plotline could be that Twilight discovers her best friend has been secretly working against her all along and planned something devious for the wedding. Maybe this friend has a hidden jealousy towards Twilight's relationship or her role in the wedding. Another could be that Twilight is betrayed by a new acquaintance who she thought was a friend, but in reality was an enemy in disguise, leading to chaos at the wedding.
One idea could be a story where Luna and Sombra meet in a dream realm. Luna, being the princess of the night, has the power to traverse dreams. Sombra, once a powerful and dark figure, could be lost in this dream realm. Luna tries to understand his past and help him find redemption. It could explore themes of forgiveness and the power of understanding.