I have an idea! I have many ideas. I have enough to keep me writing well into my 90s when AI will suck the wavelengths out of my brain and feed it to the matrix. The ideas, they multiply like tribbles. In all fairness, I've yet to meet an author who lacks ideas. Time is usually the mitigating factor, followed by contractual commitments, and the pursuit of a guaranteed income stream.
Obligations to third-parties aside, how do I decide which idea to work on first/next? Three steps:
1. Can I easily and clearly envision the Goal, Motivation, Internal Conflict, and External Conflict?
If "yes" proceed to next step...
I proceed 96% of the time
If "yes" proceed to next step...
I proceed 43% of the time
If "yes" prioritize and place in the queue to be written.
The answer to this one is usually manic cackling and a lot of "oh, hell no." Not because I'm a slow writer and so far my "best" time is 18 months. No, no, that piece of reality doesn't factor in. It's my belief that I could write it in 90 days. That belief stems from the notion that I am so familiar with the plot, all the characters, and each setting that I know how 27 of the 30 chapters are going go before I start Chapter 1.
That...that doesn't happen often, which makes it a great filter.
What I don't ask myself is "will anybody buy this," for the simple reason that I can't control what people will buy or when they'll buy it. I can control my little speck of the universe...sometimes.
