I think that for most of us there is a book the heart tells us we have to write, that we must write, and I think it is important to write that book. For some of us (like me, for instance, with my first book) it's impossible not to.
(It looks like you've already done that amw, so now you can only keep submitting it and see what happens.)
But ... in my experience, most writers have more than one idea that they really want to write about, more than one idea they can fall more and more in love with as they get further into it and get to know the characters.
So once the "have to write it' book is out of our systems (or put aside until we feel we can do it justice), I think the practical thing to do is to choose from among those other ideas the one that is most likely to find a publisher and a host of loyal fans. Of course, it's impossible to know for sure what readers and publishers will want by the time the book is written, but it's usually possible to make a pretty good guess.
(It looks like you've already done that amw, so now you can only keep submitting it and see what happens.)
But ... in my experience, most writers have more than one idea that they really want to write about, more than one idea they can fall more and more in love with as they get further into it and get to know the characters.
So once the "have to write it' book is out of our systems (or put aside until we feel we can do it justice), I think the practical thing to do is to choose from among those other ideas the one that is most likely to find a publisher and a host of loyal fans. Of course, it's impossible to know for sure what readers and publishers will want by the time the book is written, but it's usually possible to make a pretty good guess.