You wouldn’t think that there is a way to critique brainstorming, and you would be right.
Brainstorming isn’t something you critique per se since in brainstorming sessions you usually just spitball ideas until something or a part of something sticks. It’s generally a free-for-all, no-holds-barred event where everyone is just trying to help the author come through their latest writing hurdle.
I mention it here specifically because it’s important to touch on it. It’s one of the writing steps, and every step of the way in writing your story you have the possibility to collaborate, get feedback, or at very least run some ideas past others.
What I do want to touch on here is the hats you wear while doing this. You can be a reader, a writer, or an editor. You could even classify lawyer into this, and I’ll get into that here in a moment. Each of these has you approaching this process slightly differently.
As a reader: it’s important to remember that even though something might not sound appealing at first, under the care of a good writer, almost any idea can turn into a pretty good story. So be careful to not poo-poo ideas outright.
As a writer: be cognizant of what you can and cannot write well. Will this story require a deep-dive into Sub-Saharan folklore, etymology, geopolitical environment, climate, and astronomy? Are you ready to take on such a project? Will you have to invent an entire magic system, or will you base it off of something else? Think through the different boxes you might have to tick as a writer to be prepared for this assignment.
As an editor: are you planning on doing something crazy like writing the entire thing in second-person point of view? Or maybe you don’t want to use punctuation. I’ll include marketing things here too because maybe your sub-Saharan Astro-story might not sell well if the country in question is at constant war? Things to think about.
Lawyer: Are you planning on riffing on ideas from other writers? Are there ramifications for that? What if you include real people in the story? I recently put FDR into a story of mine, and all my feedback was positive, but it’s something to think about. Also, is anything in this story going to offend people? Personally, with our current outrage culture, too many people get upset over click-bait headlines, but you may not want to rock boats, or you might want to rock boats. How many puppies do you plan on killing in your story? Certain topics might color your story in a way you don't want, and now is the time to flesh them out.
Anyway, those are my thoughts on the process.