It could be a new character entirely. A mysterious pony who just arrives in Ponyville and catches Twilight's eye with his unique charm and different way of looking at magic.
Perhaps it could be an older character from the series that we haven't really considered in a romantic light for Twilight before, like a pony she's worked with on a project who she starts to see in a new way as they spend more time together.
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.
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.
Maybe her boyfriend is an ordinary pony at first glance but has a hidden talent for understanding complex magical artifacts. Twilight discovers this when they both stumble upon an ancient relic. They start to work together to unlock its secrets, and in the process, their relationship blossoms. They might face some opposition from other ponies who think Twilight should focus solely on her studies, but they prove that love and learning can go hand in hand.
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.
She could marry Spike. They have a very special bond throughout the series. Spike has always been there for Twilight, and in fanfiction, this could develop into a romantic relationship leading to marriage. They already have a deep connection, and Spike's loyalty and Twilight's intelligence could make for a great couple.
It could be Discord. He loves causing chaos and kidnapping Twilight Sparkle would be a great way to disrupt the harmony in Equestria. He might do it just for fun or to see how Twilight's friends would react.
It could be Discord. He loves causing chaos and might capture Twilight Sparkle just for the fun of it or to disrupt the harmony in Equestria.
There could be some jealous ponies in Ponyville as antagonists. They might not understand Twilight's relationship with the changeling and start spreading rumors. This could create problems for the couple, like making Twilight doubt the changeling or making the changeling feel unwelcome in Ponyville.
In some fanfictions, she could die sacrificing herself for her friends. Twilight is extremely loyal to her friends in Ponyville. So, if there was a situation where her friends' lives were in danger, like from a powerful villain, she might choose to give up her own life to protect them. This would be a very heroic and in - character way for her to die.
She could be friends with the children of Twilight's own friends. For example, Pinkie Pie's daughter might be a fun - loving and energetic friend, always getting Twilight's daughter into some light - hearted adventures.