Subplots are great tools to add depth, details, and tension to your story. They help develop your characters and show readers more about the characters, plot, and theme. Subplots should run parallel and help your main plot, not be random extra scenes.
Here are some ideas to use in your book:
Will they or won't they? This adds tension, gives readers a couple to ship, Be sure to check out my tropes post for various ways to use romantic subplots.
Love without the romantic aspect is great too- friend, family, pet, or object. Anything that can be threatened or makes the MC more well-rounded.
This can be between protag and antag, between friends, family, coworkers, teammates, etc. Conflict subplots add tension and increase pacing. Maybe characters have conflicting goals or hidden agendas. The best friend is tired of not being in the spot light, or they die trying to save the MC.
This reveals backstory about characters & your world in a showing way vs telling/info dump. This can be done through flashbacks or through a mentor or lesson.
Main Character Subplots
Maybe on the side they're battling anxiety, an addiction, or a fear that is holding them back. They could have a dream or wish, and maybe it opposes the plot they're thrown into and helps with their debate beat.
You can give them habits and traits that create subplots. Have them grow and change their view about something at the end of the book.
Side Character Subplots
Use these characters to add depth and tension. Maybe the side character knows something about the MC that the reader/or other characters don't know. Maybe they use it for blackmail or leverage, or maybe they're a loyal friend who's there to help the MC through something they struggle with.
If you're in First Person POV, side characters help add to the MC's knowledge of what's going on.
Side characters can also add great plot twists, but they need a want that puts them into the plot to begin with.
Two Worlds Collide Subplot
This can be used for a meet cute scene, or when the MC meets a mentor when they've just learned they have powers, or when the Antag and Protag are actually siblings (plot twist!), or someone from the MC's past shows up for good or bad. If you're stuck, this is a really fun subplot to use.
The two worlds can also be the opposite view points and help the MC learn to think about things differently, and grow. An example of this is Renegades when Nova and Adrian meet and they've been brought up to believe different things about the 'heroes'.
Your characters have to go on an adventure to save the world, but they get split up along the way. Oh no! The main plot is saving the world, but the subplots are how the split teams are handling the situations that are thrown at them.
Maybe one group has become imprisoned. The other group is traversing a mountain with billy goat vampires. And your MC's group just found a traitor in their midst. Dun dun dun. All these subplots add tension on top of the tension of 'Will they ever get back together? How will they save the world?'
When using the Save the Cat Write a Novel Beat Sheet, you start with the Opening Image & Set Up. This shows the Main Character in their home, work, and play life, all of which can be a subplot to the main plot of that MC trying to achieve their wants/needs.
Home, work, and play, add depth and complexity to your story.
In that same Beat Sheet is the B Story in Act 2, when a character enters the MC's life, for better or worse. The B Story is a subplot. From there you can have C Story, D Story, etc, depending on your characters and their relation to the plot.
Subplots are also great for upping word count, but be careful that you don't add too many subplots.
Do you have any favorite subplots?
I tend to use friendships and love interests, as well as some mystery or thing the MC needs to learn.