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.
Maybe in the fanfiction, Luna adopts Twilight because she senses that Twilight is being overlooked in some aspects. Luna then helps Twilight gain more confidence and they embark on adventures together, facing challenges that test their newfound relationship.
A plotline might involve Luna seeing Scootaloo's potential and deciding to adopt her as a sort of protege. This could lead to Scootaloo learning about Luna's powers and responsibilities in Equestria. For example, Luna could take Scootaloo on nightly adventures to teach her about the magic of the night. Along the way, Scootaloo could also help Luna reconnect with the ponies who might still be a bit wary of her due to her past. It could be a story of growth and friendship for both of them.
The main characters are clearly Luna and Twilight. Luna, being the one who adopts Twilight, is likely to play a guiding and protective role. Twilight, on the other hand, is the one being adopted and will probably experience growth and development under Luna's care.
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 possible plotline could be that Luna sacrifices herself to save Equestria from a powerful and ancient evil. She might use all of her magic and strength to defeat the foe, but in the process, it drains her life force completely.
Well, sometimes in these fanfictions, a great plotline is Snape trying to protect Luna from the bullies at Hogwarts. Given Snape's own experiences of being bullied as a child, he would be very determined to make sure Luna doesn't go through the same. He might use his position as a teacher to subtly intimidate the students who pick on her. Also, there could be a plot where Luna starts to teach Snape about the beauty of the natural magic around them. Snape is so focused on the academic and often darker aspects of magic, that Luna's view of the magic in simple things like a butterfly's flight or the way a flower blooms could be eye - opening for him.
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 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 plotline could be Goku finding Twilight Sparkle in a strange new world and taking her under his wing. He might train her in his fighting techniques, which could be really interesting as Twilight is used to magic - based solutions.
Friendship is a very common theme. Since Twilight and Luna are both important characters in MLP, their friendship is often explored, like how they support each other through different challenges.
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.