It could be Abuela Alma. Since Mirabel getting a gift would have a huge impact on the family, Abuela's reaction and her journey of accepting this new change in Mirabel could be a major part of the story.
Perhaps in the fanfiction, Mirabel gets a gift that allows her to see the true potential in each family member. So she uses this gift to encourage her family members to embrace their individuality more, which in turn strengthens the family bond. For example, she might show Luisa that there's more to her than just strength, and help her find a new passion.
The concept of 'neglected Mirabel' in fanfictions also stems from the fact that her character has so much potential for growth and development. By imagining her being neglected, fans can write about her journey to prove herself, find her place in the family, and perhaps even change the family dynamics. It's a way to delve deeper into Mirabel's character and the relationships within the family that we only got a glimpse of in the movie.
Perhaps she tries to understand Abuela's perspective. Mirabel is a kind - hearted girl, and she may think that there is a reason behind Abuela's actions. So she might try to see things from Abuela's point of view while still feeling the pain of the punishment.
Mirabel might be initially shocked and heartbroken. She loves her family and the village, so being banished would be a huge blow. She could cry and try to plead with Abuela to change her mind.
Perhaps Abuela believes that Mirabel's presence without a gift is somehow jinxing the family's magic. In the fanfiction, she might think that by imprisoning Mirabel, she can stop the negative effects she thinks Mirabel is causing. This could be related to some superstitious beliefs within the Encanto about the balance of magic and the role of each family member.
At first, Mirabel might be shocked and confused. She would probably wonder why Abuela would do such a thing to her. She could spend her initial time in prison trying to figure out what she did wrong or if there was some mistake.