Self - discovery can be a theme as well. The reader, through their experiences with the AoT girls, may learn more about themselves, their strengths, and their weaknesses. It's a way to explore the character's growth within the unique context of the Attack on Titan universe.
It's fairly common. Fans often look for more content related to their favorite shows. Since 'Attack on Titan' has a large and passionate fan base, there are a lot of fanfics available. 'Aot' fans might watch (read) these fanfics to explore different scenarios that the official story didn't cover. For example, they might want to see how a certain character would react in a different situation or what would happen if a major plot event had a different outcome.
There could be various fanfictions. Maybe some that focus on alternative endings, like ones where certain characters survive who didn't in the original. For example, a fanfiction where Erwin survives and leads the Scouts in a different way.
In addition to the main lost girls, there could be side characters such as wise old men or women who offer advice to the rescuers or the lost girls themselves. There might also be animals in the story. If it's set in a forest, for example, friendly animals could help the girls, or dangerous animals could add to the challenges they face.
It could be about girls who are lost, perhaps physically lost in a place like a forest or a big city. Maybe they are lost in a more metaphorical sense, like lost in life, not knowing their purpose or direction.