I don't have pre-production because I start with the real production directly. But, figuring out story, characters and atmosphere has taken about a year (that's how long my whole game to make from first idea to finish). Lots of things fell into place the last day, kind of. Brainstorming ideas is easy: Do it regularly, think like a kid, choose a theme (it's easier to come up with 5 ideas based on "football" than just 5 general ideas), write down everything that feels wrong, ask your friends, don't be a perfectionist, then choose the best of all these dumb ideas... If you want to know any specifics, you can ask me on DM, if that exists on itch.io?. I have so many thoughts and ideas on this that I don't know where to start in this reply...
Even if you write a draft you'll change a lot of things when you see how the things look in real production. So get started with production as soon as possible? Maybe? I don't know, you do it in your own pace.