Once the murder has been solved, what do we do? If it had been planned to last longer, or something goes wrong, what should I do during the extra time?

Ideally, characters should have secondary goals to ensure that not only are they trying to find out what happened, but they also have an angle to interact with one another. The old rich uncle everybody hated died under mysterious circumstances. Everybody wants to find out how, but also Wilma needs to make sure her name is on the will after her very public fallout with her father, Sally needs to find a way to tarnish the old man's memory because he was abusive, Jack wants to hide or destroy evidence that he was blackmailing his employer after witnessing said abuse, etc etc. The more intertwined your stories and characters are the better