Brainstorming is my favorite part of story writing. I think it’s possible, if you are the outlining kind of writer, to create a pretty strong story before you start writing by simply asking the right questions.
You know when you go see a big sci fi or fantasy epic at the movie theater and (occasionally) you spend the whole time punching holes in the plot? “Why didn’t they just call the eagles to take them to the Lonely Mountain?” “If she can blast people with her magic, why did she spend so long running away from the other witch?” So when you’re brainstorming ask yourself thorough questions. Rather than impeding creativity, I find it boosts mine because problem solving is a creative exercise. ‘What if’ is fun, and a great place to begin brainstorming, but don’t neglect ‘why,’ and ‘how.’
Another brainstorming method I’ve used effectively is to take a notebook (the catnip of writers), open it to a blank page, and do a free write where you list every cool or inspiring thing that comes to you. Sometimes by combining story elements you think are awesome, you arrive at new ideas and characters and plots.
If you use this method, I’d suggest you spend some time towards the end using with the first method I discussed. Ask yourself as many questions as possible. Explore all possible ramifications of the story you’ve come up with. Or don’t. Maybe you’ve written enough to realize that you like exploring your plot tangles while writing the first draft, i.e., a gardener or discovery writer. That’s cool too.