In a writing book I read recently, the author suggests doing a bunch of list-making before you start to plot out the story. The first list should include every idea you can envision for the story you're about to write. You'll include everything, no matter how ridiculous it seems or how unlikely it is that you'll use it. Doing this will release the "censor" in your brain and allow your subconscious to run wild to generate ideas.
Next, you take that list and filter it down to the ideas you can imagine actually using in your story. For those items, you brainstorm around them also. Ask yourself questions like: If this really did happen in my story, what might happen to cause it? What might happen as a result of it? What repercussions would it have on the other events I've planned? The idea is to consider all the ways the event might impact your story and all the other possible events it might spark in your imagination.
When you exhaust your imagination on those events, you start a third list. For this list, you try to imagine every expectation your reader might have for the story you're writing. For a science fiction story I am working on, I tried to compare this story and the universe it's set within to every other science fiction universe I could remember. I asked questions like: What would the reader expect to happen if this story was in the Star Trek universe? What would they expect based on sci-fi television episodes that are similar to this idea? Ignoring science fiction, what expectations might they have about the characters, the situation, etc.? The point of this list is to identify as many of your readers' expectations as you can. You'll use the list two ways. The first use is to consider which of these expectations you intend to deliver on, and which expectations you definitely won't deliver on. The first side of the exercise helps you think about things you need to do in the story to ensure that the reader is comfortable with it. The second side helps you find ways that you can surprise the reader or show them that your story (or story universe) is not the same old thing they're used to. Those surprises are likely to be the things readers will enjoy most in your story.
Now, when you sit down to plot your story, you'll have lots of possible events to use, you'll know what reader expectations you plan to deliver on, and what you plan to do to surprise the reader.