One interesting rent a girlfriend fanfic could be about how the rented girlfriend and the client start to develop real feelings for each other during their pretend relationship. They might go through various situations like meeting each other's families in a fake - relationship - turned - real - feelings kind of scenario.
First, create interesting characters. Give the rented girlfriend a unique personality, like she could be really shy but pretending to be bold for the job. Also, give the male lead some quirks. Then, think of a good plot. Maybe they get into a situation where they have to keep up the act in front of the client's ex - girlfriend. This will create lots of drama.
I'm not sure exactly when it's ending. You might want to keep an eye on the official announcements from the publisher or the manga creator's social media.