I prefer her being random. Being Obi-Wan’s granddaughter would feel forced. I don’t see how a reveal of such information about her lineage would change the story all that much. When Luke finally figured out Vader was his father, it affected everything. He was just lied to. Luke had already built up the image in his mind of a valiant warrior who was his father. Figuring out that the biggest baddie in the entire galaxy is your father is much more than figuring out your grandfather was just a noble Jedi. If they revealed Obi-Wan to be Rey’s grandfather it would be as if we never got ANY hint as to who Luke’s father was in episode 4, and then in episode 5, he figures out his dad was a good pilot and a friend of Obi-wan. It wouldn’t affect Luke all that much or the story. That’s why her being random or Luke’s daughter would be fine. Because it can tie into her childhood and actually affect her character in a meaningful way.