The short answer is none of the above. I have to decide if it's a project I think I can sell to a publisher. Some ideas have been wonderful, but the reading audience for that topic is too small. Or there are already too many books on the topic. These days, personal experience books are difficult to sell; fiction is also in a heavy downswing.
If the idea sticks with me, I play around with it until some form or structure comes into my head.
The one thing that hasn't failed is this: When I know the first sentence, I'm ready to write. (I may edit that sentence five times, but it tells me where to begin.) Once that happens, the book begins to unfold and takes shape.
I want to add that I'm a runner. Usually the idea turns around inside my head, and one day when I'm hitting the pavements that first sentence pops out. Then I know I'm ready to begin. This is just as true whether it's my own book or a manuscript I'm writing for someone else.
For me, marketing is one of the last things I consider—other than asking myself if I think it will sell. I'm absolutely terrible at marketing; however, I have a wonderful virtual assistant, Twila Belk, who intuitively knows more about marketing than I could ever learn. So after I write the proposal, I ask Twila to help me. (Please don't tell her; I don't want her to think she's important.)*
The above paragraph is to say that I know what I do best: I write. I could spend much of my efforts in marketing strategies, which I did badly—in the past.
I focus on what I do best;
I get help from those who do what I can't do effectively.
* * * * *
* A note from Twila: Mark this day in history. Cecil Murphey said something nice about me. Wow! But don't tell him I know. He doesn't think I can read. (Twila smiled.)