I think that is the key, to not wait a single moment longer than necessary, because most ideas are so very insubstantial (is that the word I’m looking for, don’t think so, no) at the very first stages that they vaporize into thin air as soon as they are hit by daylight or any kind of reality check. Just think the words ”Who would want to read a book about _____?” and poof, the idea is gone.
No, write it down first. Ink on paper or black pixels on a white background or whatever. That gives the ideas a chance to solidify and mature. Like putting your seeds on top of a little potting soil, in a quiet corner of the garden, instead of just scattering them in the middle of the road or wherever.
And then, don’t bother them for a while. Just let them be. If you keep watching the pot, nothing will grow. If you keep staring at this list, nothing will get written.
Just read through it once in a while, and see if something pops out at you. Perhaps something that makes you think Did I write this? What on earth was I thinking?
Those ideas are often the best ones.