First, you can start with his appearance. He could look similar to Twilight but have some distinct features, like a different color in his mane or tail. In terms of personality, he might be more outgoing than Twilight. For example, he could be the type to easily make friends with everypony he meets. This would contrast with Twilight who can be a bit more reserved in new social situations.
One idea could be that the twin brother has a different magical specialty. For example, while Twilight Sparkle is great at magic related to friendship and harmony, her brother could be more attuned to magic of the elements in a different way, like having the power to manipulate natural elements more directly. Maybe he could use his magic to help protect Ponyville from a new, different kind of threat that Twilight's magic isn't as effective against.
You can start by giving her a distinct personality. If Twilight is studious and a bit reserved, make the sister more carefree and impulsive. Maybe she has a passion for art or music that sets her apart. Her appearance could also be different in a subtle way, like a different shade of mane or a unique cutie mark.
Celestia and Luna might also show up in some fanfictions. Celestia could offer wisdom to Twilight Sparkle's little brother, or Luna might appear in his dreams to give him hints about a mystery he's trying to solve. Their presence would add an element of grandeur and magic to the story, connecting the little brother's small - scale adventures to the larger world of Equestria.
One idea could be that the twin sister has a different magical ability. For example, while Twilight Sparkle is good at spells related to knowledge and friendship, her sister could be excellent at magic related to nature. This would create a contrast and also opportunities for them to learn from each other. Another plot could be that the twin sister gets into trouble in a different pony world and Twilight has to go rescue her, leading to all kinds of adventures in unknown places.
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.
One interesting storyline could be Twilight Sparkle's little brother getting involved in her magic studies. He might accidentally cause some chaos in her study area with his own budding magical abilities. For example, he could make all the spell books float around in a disorganized mess while trying to help Twilight organize them.
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.
It could be a pony who is extremely extroverted if Twilight is introverted. Someone who loves chaos and spontaneity instead of Twilight's love for order and planning.
One possible fanfiction could involve Twilight Sparkle's dragon transformation being a result of a powerful magic spell gone wrong. She might struggle to control her new dragon form and powers at first. For example, she could accidentally set things on fire when she tries to use her magic in her dragon state. As the story progresses, she learns to master her new form through the help of her friends.