She was sent away from her kingdom when she was young, so she wouldn't be taken prisoner again by King Sombra. She grew up away from the empire. Now, she's the one who has to save it. First from Sombra, then from Chrysalis.
The birth of Flurry Heart was a bit more complicated than first thought. When she woke up the next day, it was apparent that she may or may not have had a life before she was Flurry Heart.
Flurry Heart has many things on her mind like when she'd next spend time with Twilight, and how she'd go about befriending somepony as vile as King Sombra.
Flurry Heart has requested a very unusual bedtime story. Shining Armor senses something is up, but he also knows that children occasionally come across feelings they don't have words for yet. Books can supply those words.
An older, mellower Scootaloo relocates to the Crystal Empire, tasked with tutoring a jumpy junior Princess. She's either the worst pony for the job or the ONLY pony for the job. There is no in-between.
As a budding diplomat, Flurry Heart often travels out of Equestria and into other dimensions. It's not uncommon for her to get stalled in the Between Place, but something is different this time. Someone else is here.