I'd say these wise words x100.I don't think of it in terms of which will sell or bomb as that's really hard to predict, but more in terms of which one I want to write most. Right now I have ideas for 6 books, and I don't let that initimidate me - instead, I just sequence them and go after the first one that fits my parameters (most want to write, best fits my series narrative etc.)
I have a document that is strictly just story ideas. I dump everything I can think of into the document, then close it, and sit on it until my thoughts settle. Once I have a clear head, I then plan if I'm going to move forward with it. I get what you mean, though, because I was halfway through writing Of Fury and Fangs and got an idea for a better novel. At first, I tried to integrate it into this book, but it didn't mesh well, so I extracted the 11k words I'd written about it and revised the entire thing. Now that's done and published, so I can tackle the new idea in the next book. The other thing I do before I actually begin a novel is not an actual outline--it's a list of plot bunnies and ideas that COULD go in the book, but I review each one to see if they gel with each other and make sense. I also have to admit I kinda don't care what sells? I write for myself first. If other people like it, whoo hoo! That's great! If not, oops, sorry. That being said, I do make sure to check my work against stereotypes, cliches, and tropes to be sure my work isn't too narrowly focused.
There's an important distinction between a good idea and a good book. Not everything that you think to yourself, "That's awesome! What a great book that would make." will flesh out.
I write up any ideas that interest me in longhand in a notebook, using the last few pages of the notebook as a table of contents. Alot of them fall by the wayside. I lose interest in them. I don't care that I never got around to writing them, I don't care if they maybe had more commercial appeal than the stuff I actually wrote. Some keep cropping up in different variations until I find one that works for me: the space opera novel I released around Christmas 2020 was written in 2019-2020 but grew out of several previous failed novel attempts going back to 2014 or thereabouts.