One idea could be a fanfiction where the characters from 'Pretty Little Liars The Perfectionists' time - travel to a different era. Maybe they end up in the 1920s and have to deal with the social norms of that time while also trying to figure out how to get back to their own time.
A fanfiction could also be about a character from the show getting superpowers. For example, Alison could develop the power of telepathy. This would completely change the dynamics among the characters. She could use her power to solve the mysteries more quickly, but it could also cause problems as the other characters might be afraid of her new abilities.
Another interesting fanfiction could focus on an alternate universe where the main conflict in the show never happened. The characters are leading normal, carefree lives, but then strange things start to occur that seem to be remnants of the original conflict, forcing them to face a new mystery.