Well, in such fan fiction, a theme could be about overcoming challenges. Maybe Twilight Sparkle has to deal with new limitations or advantages because of her new body type. There could also be a theme of mystery, like if her change to being 'fat' is part of some larger mystery that she has to solve. And of course, there could be themes related to the magical world she lives in, such as if there are spells or potions involved in her change and how she uses her magic to either deal with or reverse the change.
I'm not sure specifically which 'fat twilight sparkle fan fiction' is popular. You could try searching on fan fiction websites like FanFiction.net or Archive of Our Own. They have a vast collection of fan - made stories related to various characters, including Twilight Sparkle, and you might find ones that fit the description there.
A common theme is self - acceptance. Twilight Sparkle has to come to terms with her new body. Another theme could be friendship. Her friends' reactions and how they support or not support her through this change.
One possible theme could be sacrifice. Maybe Twilight Sparkle sacrifices herself to save her friends or Equestria. Another theme might be dealing with grief among the other ponies after her death.
A plotline might involve Twilight Sparkle being transported to a Spartan - based realm. There, she has to prove herself in their combat trials. As she progresses, she discovers a conspiracy within the Spartan society that she has to expose using her intelligence and magic. Or perhaps she teams up with a group of Spartans to retrieve a powerful magical artifact that is of great importance to both her world and the Spartan world.
Well, obviously Princess Cadance and Twilight Sparkle are the main characters. Princess Cadance is known for her role in ruling the Crystal Empire and her powerful magic related to love. Twilight Sparkle, on the other hand, is a very studious and powerful unicorn - turned - alicorn. She is often the one seeking knowledge and using her magic to solve problems.
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.
One possible plotline could be that Twilight Sparkle is kidnapped by a powerful dark magic user who wants to steal her knowledge of magic. Her friends then embark on a quest to rescue her, facing various challenges along the way. Another could be that she is kidnapped by an unknown force from another dimension for some sort of experiment. Her friends have to figure out how to cross dimensions to save her. A third plotline might be that a group of villains kidnap Twilight Sparkle to use her as a bargaining chip against Equestria, and her friends must find a way to outwit the villains and get her back safely.
One possible plotline could be Twilight Sparkle dealing with the physical and emotional changes of pregnancy while still having to fulfill her princess duties in Ponyville. Another could be her friends helping her prepare for the arrival of the new foal, like Pinkie Pie throwing a baby - themed party.
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.
One possible plot could be that Twilight Sparkle is captured by a group of dark magic users. They might be after her powerful magic or knowledge. Her friends then embark on a rescue mission, facing various challenges along the way.