What is Interesting About It?

When you have a cool story idea, you should stop and figure out "what makes it cool?"

Let's say your story is "Byzantines land in the New World"--what makes that cool? Partly it's the sense of wonder from discovering a new world--the new cultures, the new landscapes, the mysterious ruins, the dinosaurs. But why the Byzantines? Because you want to emphasize, not brute force, but cleverness, subtlety, making allies of the natives (which the Spanish did, but most people don't know that).

Let's say the story is about a modern man who can see the Hidden World. In that case, make sure there's plenty of Hidden World stuff to see.

A spy in the Napoleonic era? Pump up the James Bond-ness and the history.