One's Next Project

I'm assuming (perhaps wrongly) that I'm not alone in my current conundrum: what to write next.

It's not that I lack for ideas. If anything, there are too many. But that, for me at least, has always been the case.
But no, the lack or presence of ideas is not what I'm pondering. It's the proper order of things. When I consider what project to work on next (beyond the editing/formating/publishing of works which are already in essence complete) I have to consider a number of factors.

1) What I'm interested in. Not all ideas are created equal, and some of what I could write, I don't really want to. I can't picture sustaining my own attention over the amount of time (a month, three months, what have you) that a first draft would take me. Likely I would prove myself wrong: almost anything becomes interesting once you're really working on it. But that's the problem. I would have to start working on something, and it had better, at least at first, interest me.

2) What I think has value, that is, what I could sell. Pet projects are nice, and there are some things I've written that I know are just too particular/strange/pointlessly attuned to me/whatever that I'm certain there's no market for them. This didn't used to be a problem. I wasn't selling anything at all, and I didn't have to worry about whether something would sell. Now, of course, I do sell some books (not a lot, but see my next reason), so I actually have to think about what could sell when I'm all done with it.

3) What I think not just could, but will sell. I have some notion, faint as it is, of what I am selling. So I have some idea that if I produce something similar, such a thing might also sell. Which means it's not just whether I'm interested, and whether other people could be interested, but whether they in fact have shown an interest.

So I have to decide: do I write something new that I find interesting but that is totally unproven? (I've got two of those that I could start working on right away.) Do I write something connecting to something that may or may not have any real value because the first book isn't necessarily doing great? (I've got one of those.) Or do I go with something that my evidence suggests will do the best, but which, frankly, I've tried working no before and not really liked.

The one thing I know is, after some months of mostly editing/formatting/cover designing/etc, I've really got to get back to writing on a longer project, and promptly. So I have to make the choice, and soon. But not, I think, today.