Having read Jennifer's comments, which I think make sense, I should qualify my statement.
When I wrote my first book, I believed in it enough to write it entirely on spec. I devoted a huge amount of concentration and effort to the project with absolutely no guarantee of any return at all. But I did some things on my side . . . my brother was already a huge success, and I knew his agent would read my work and offer advice.
And the gamble paid off. My bound galleys will be available for advance readers in just two short months, and I'll be in bookstores everywhere this fall.
So now I consider book two. Book one gave me many ideas for a sequel. The public response gave me ideas. The responses my publisher received gave me ideas. And my agent told me what he thought of them.
So I am not making topic decisions in a vacuum. When I say, "I'll write a book and then show it to the publisher," I already have an excellent idea what they want. Sure, I will do plenty of rewriting and editing, but I'm not just going on a wing and a prayer.
But book #1 - that's exactly what it was. Could I tell a story in a compelling enough way to interest a publisher? Who could know? So I guess you should consider both our comments in light of your own position.
I don't want to encourage you to go off on a wild goose chase. You should think very carefully about how well you know your publisher, your market, and your abilities.
For me, the thing I doubt most is my own ability. Maybe I shouldn't, but there it is. So I will address that by writing most of the book first. Then, if it's good enough, we can go ahead and I'll feel good, and not pressured. I would never want to sell a project and then discover I could not do it. I prefer to do most of it first and then sell what I've done.