Originality isn't required. Seriously. Classic mystery plots have survived the test of time for a reason. So plagiarize away - or
steal one of these ideas. Just make sure to flesh out your theme by following these steps:
Choose a fun setting. Is your escape room taking place on a train, a plane, or a yellow submarine? Have you relocated your guests to the Everglades or to Yellowstone? The sky's the limit! Which, come to think of it, means your game could take place on the International Space Station!
Have clear goals... The goal is not simply to escape. That's boring. The players have to uncover the identity of the murderer. Here's where you up the ante. You must stop the killer because she leads a spy agency, poisoned the town's water, or stole all the girl scout cookies. Motivate them with an irresistible goal!
... and consequences! The players also need to know what happens if they fail. Will they be killed next? Will the killer escape in a time machine to rewrite the ending to Casablanca? Will their google search history be posted online? Make sure they know the price of failing to uncover the killer. You want them to be properly motivated.
Optional - Add a cunning twist. Be careful here because twists don't always work out. If you've incorporated non-player characters into the game, throwing the players a wicked curveball is easier. Maybe the NPC is the killer or the evil mastermind behind the killer. If the NPC has been providing the players clues, give them information that makes them distrust the NPC. You can imagine all sorts of ways to have a great twist ending to your game.
Keep the twist simple. If you can't describe the twist in two sentences or less, try again. Remember, the players only have an hour to escape. Complicated last minute twists aren't fair or fun.