One idea could be exploring the family secrets of the Liars' parents before they were born. Maybe there was some kind of feud or alliance between the families that the Liars are unaware of. Another could be a story where one of the Liars discovers they have a long - lost relative who has a connection to 'A'. For example, a cousin who has been watching over them all this time.
Well, a great plot could be that Caleb finds a strange encrypted file on his computer that seems to lead to a hidden treasure in Rosewood. As he tries to decipher it, he has to team up with some unexpected characters from the show. Maybe he also has a vision or a strange dream that leads him to believe there's something supernatural going on in the town related to the file. Also, he could get into a situation where he has to choose between his loyalty to his friends and his own personal gain from the treasure.
One idea could be Aria exploring a new art form in Paris for a semester and getting involved in a mystery there related to the 'A' games. Maybe she meets a mysterious artist who seems to know more about her past than she does.
One idea could be Haleb going on a secret adventure together away from all the drama in Rosewood. They discover a hidden place that has a connection to one of the mysteries in town. Another could be them dealing with family issues together and growing even closer through it.
One idea could be that one of the main characters discovers she's pregnant and has to keep it a secret from the 'A' threats. She might be afraid that 'A' will use the pregnancy against her or her friends.
One idea could be that Paige and Emily go on a road trip together to discover more about their relationship away from the drama of Rosewood. They could encounter various challenges along the way that test their love and loyalty to each other.
Another plot could be Mike developing a relationship with one of the new characters in town. This new relationship could be complicated by the secrets and drama that always seem to surround Rosewood. For example, the new character could be related to one of the 'A' suspects, and Mike has to figure out if he can trust them.
One idea could be exploring a different backstory for one of the main characters. For example, delving deeper into Alison's life before she disappeared. Maybe she had a secret friendship with someone outside of the main group that no one knew about. This could add a whole new layer of mystery to the story.
One idea could be a character getting a drastic haircut as a form of rebellion against the 'A' mystery. For example, Spencer might cut her hair short to show she's taking control of her life instead of being constantly tormented. Another could be a character getting a haircut to disguise themselves from 'A'. Like Hanna changing her look completely with a new haircut to avoid being recognized while trying to figure out who 'A' is.
One idea could be exploring the backstories of the minor characters in more detail. For example, what was the life of Jenna before she became blind? Another idea is to create an alternate universe where the 'A' mystery was solved much earlier, and how the characters' lives would be different. Maybe they would all be closer friends and focus more on normal high school stuff like prom and college applications.
Another plot could involve Alex Drake having a change of heart. Maybe she starts to feel guilty about her actions and decides to help the liars against her former allies. She could reveal some crucial information about 'A' that she had been withholding. This could lead to a tense situation where she has to prove her loyalty to the liars while still being feared by them because of her past deeds.