Take any historical event you can think of. It could be ancient history, modern history, Greek history (helloooooooo Brad Pitt in Troy), or something that happened in your hometown six years ago. Mix it up in a pot, and you have yourself a plot (I didn’t mean for that to rhyme, I guess that’s just the magic of writing).
Reinventing history is a popular writing trick that a lot of filmmakers turn to for creative ideas and inspiration; you can take a historical event and change it up in any way you'd like. From unsolved murders to the first black LAPD officer, any real-life event can be retold in any way you choose to tell it.
Take a look at Truman Capote when he wrote the first Non-Fiction novel, In Cold Blood. He wrote one of the great American novels through the first-hand experiences of those involved, despite conflicting accounts of the murders. Ironically enough nearly 50 years later Bennette Miller created the Oscar-winning movie Capote from the experiences Truman Capote had while researching the Clutter Family Murders.
You have creative license to write about the subjects that you want to write about, whether they’re based, inspired, or are true stories buried behind an allegory. The universe and all the stories within it are yours for the writing. The only rule is… “don’t be boring!”
There are a lot of stories on Wikipedia, from pioneers to people you’ve never heard of, key events, murders, disappearances, government conspiracies, and much, much more. There is plenty of source material out there. You've just got to find it and have a good reason for writing it.