A great 'Twilight Sparkle immortal fanfic' could be about how she outlives all of her friends. It would be a rather bittersweet story, with her having to carry on their legacies. She might struggle with loneliness at first but then find new purpose in mentoring generations of young ponies, passing on the wisdom she gained from her long - lived experiences.
Some fanfics focus on the relationship between Twilight Sparkle and Dusk Shine. For example, a story could explore how they become friends. It might start with them being initially at odds because of their different ways of using magic, but then they gradually come to understand and respect each other through a series of events.
Some specialized My Little Pony fan communities or forums might also be a good source. Members often share and discuss their favorite fanfics. You can ask around in these communities if anyone knows of any good 'Twilight Sparkle immortal fanfics'. Just be sure to follow the community rules when posting.
One great 'Twilight Sparkle Daughter' fanfic could be 'The Adventures of Twilight's Daughter'. It often shows the daughter inheriting Twilight's magical abilities and going on quests to save Equestria. Another one might be 'Twilight's Daughter: A New Generation' which focuses on how she copes with living in the shadow of her famous mother while trying to make her own mark in the magical world.
One popular 'Twilight Sparkle futa harem' fanfic might be 'The Magical Bonds'. It explores unique relationships and adventures in the magical world. Another could be 'Twilight's Harem Quest' which has an interesting plot where Twilight navigates her new relationships. There's also 'Futa Twilight's Enchanted Harem' that delves deep into the character dynamics within the harem.
Friendship is a very common theme. Given that My Little Pony is all about friendship, many fanfics explore how Rarity and Twilight Sparkle's friendship develops and overcomes challenges.
One great 'human breeds Twilight Sparkle' fanfic could be 'Twilight's New World'. In this fanfic, Twilight Sparkle is reimagined as a human in a modern-day high school setting. It explores her relationships with other human versions of the Mane 6 and how she adapts to this new world. Another one might be 'Human Twilight's Adventure', which takes her on a journey through a magical realm while still being in her human form. There could also be a fanfic where human Twilight Sparkle discovers she has a special power that only she can use to save her friends.
One common theme could be the anticipation and excitement of new life. It might show Twilight Sparkle preparing for the arrival of her baby, like decorating a nursery or getting baby supplies. Another theme could be how her friends support her during the pregnancy, showing the strong bond within the group.
There's a fanfiction titled 'The Succubus and the Scholar'. In it, Twilight Sparkle, as a succubus, uses her intelligence to help the other ponies. It combines elements of her studious nature with the allure and mystery of a succubus. The story often has her using her powers to solve problems that normal ponies couldn't.
Some 'Twilight Sparkle fanfic' are set in alternate universes. In one such fanfic, Twilight could be a human in a world where magic still exists but is very different from Equestria. This allows for a whole new set of challenges and character development as she tries to adapt to this new world while still being true to her core self as a learner and a friend.
One popular aspect in such fanfics could be the corruption of Twilight's character. Maybe she's influenced by a dark magic or an evil entity. For example, she could start to use her knowledge of magic in harmful ways, like turning against her friends or trying to take over Equestria. Another idea might be that she becomes jealous of other ponies' powers and uses her intelligence to plot against them.