Requiem
Although the series finale Requiem DID tie up the storyline, it also left things open for a possible spinoff series. In the last few episodes we clearly see that Caleb is being drawn to the dark side by Lucas, but in the finale he is "saved" by Merlyn as she risks her "afterlife" and is absorbed into him. However, if a spinoff were to come out they could have Caleb older and the new sheriff of Trinity. The series could be about Caleb trying to fight the evil inside him thanks to being Lucas' son, and the remains of Merlyn's essence that was absorbed into him as a child.
One other thing- In the same episode when Lucas is holding Caleb over his head, he yells down to Merlyn at the bottom of the stairs and calls her "little sister". I don't remember the series ever mentioning that Lucas and Merlyn were related. Was that just supposed to mean that she "became" his sister because she's an angel and Lucas is basically the show's version of Lucifer who is a fallen angel in the bible?