Her relationship with her friends would likely become strained. They would be shocked and scared of her new demon form. For instance, Fluttershy might be too afraid to approach her. Pinkie Pie's usual bubbly nature might be dampened as she tries to figure out how to deal with this new Twilight.
She may face some challenges in adjusting to her new form. Firstly, she'll have to get used to her new size and strength. She might accidentally break things or knock over her friends. Also, she'll need to figure out how to communicate effectively. Dragons might have a different way of expressing themselves, and Twilight will have to learn that while still being able to convey her thoughts to her pony friends.
One common theme is the struggle between good and evil within Twilight Sparkle. Since she's a demon in these fanfictions, she often has to deal with her darker nature while still having some remnants of her original self.
Her curiosity might become even more intense. As a dragon, she has new abilities and a new form to explore, so she'll be eager to figure out all the ins and outs. For example, she'll be curious about how her dragon body works mechanically and magically.
A likely candidate could be Discord. He is known for causing chaos and mischief. He might pretend to befriend Twilight Sparkle only to betray her at a crucial moment. Another possibility is Trixie. Trixie has been jealous of Twilight in the past and could betray her out of envy.
Flash Sentry could be a likely groom. He has shown an interest in Twilight in the past and they have had some interactions that could lead to a romantic relationship. Another option could be a completely new character, perhaps a powerful wizard or a prince from a far - off land who Twilight meets during one of her adventures and falls in love with.
At first, her friends would likely be in denial. They've known Twilight as a kind and good - hearted pony, so they might think there's some external factor causing her strange behavior. For example, Pinkie Pie might try to cheer her up with parties, thinking that would snap her out of it.
One possible plotline could be that Twilight Sparkle is exposed to some dark magic that gradually corrupts her and turns her into a demon. As a demon, she might struggle with her newfound dark nature while her friends try to save her. For example, they could search for a way to reverse the transformation by looking for ancient spells or artifacts.
You can try fanfiction platforms like FanFiction.net or Archive of Our Own. Just search for 'Demon Twilight Sparkle' on their search bars and you'll get a list of relevant fanfictions.
Fanfiction about Twilight Sparkle being banished can also be a vehicle for exploring different relationships. Maybe she meets characters she would never have interacted with in the show. Her banishment could bring out different sides of her personality. She might become more resourceful or develop a stronger sense of self - reliance as she tries to navigate her new situation.