There is no one answer to this question since there are an endless amount of possible plot ideas out there. However, some general tips for coming up with a good plot idea include:
-Start by brainstorming a list of potential ideas. Don't worry about whether or not they're good at this stage, just get everything down on paper.
-Once you have a list of ideas, start narrowing them down by thinking about what kind of story you want to tell and what would be the most interesting to write and read.
-Once you have a few ideas that you're really excited about, do some further research to make sure there's enough material to flesh out a full story. This might include reading similar books or articles, watching movies or TV shows, or talking to experts on the subject.
-Finally, once you have a solid idea, start planning out the details of your story, including the characters, setting, and plot.