We've known her since season 2 and would have been far more invested in her than in Holly, who we knew nothing about going into season 5. They wouldn't have had to retcon her age *cough*, as she would have been around 12 years old in '87, same as Will when he was taken. She would have had a future-sister-in-law dynamic with Max in Camazotz. She could have relayed so much information to Max about what's been going on while she was gone. We could have seen her nerd out about A Wrinkle in Time and science (which is canonically her favourite subject). We could have seen actual depth added to her character and given an active, emotional storyline to Lucas, who would have been hellbent on finding her. It would have been so satisfying to see that dynamic since we're used to them bickering all the time; a real true-to-life example of the complexities of sibling love. It could have been so, so good.