When I first began writing, my primary concern was finishing the novel, actually telling a story that made some sort of sense from Page One through The End. Many years have passed, and many novels have been written - some published, most not - and now my worry is not that I can't finish a book but that I haven't chosen the right story to tell.
You probably have dozens of stories in your head, a hundred characters you could write about, and you're probably thinking that everything would be awesome if you could just find the time to write them all down.
No, that's not the problem.
The most important decision you can make is not which one to write FIRST in your limited free time, but which one to write AT ALL. Is it the vampire-zombie romance? The political thriller? The cozy knitting mystery? Or a memoir of your days working in the Peace Corps?
You may be tempted to choose the story that would sell right this very second if it were in the market. Don't choose that one. By the time you write yours, edit it, rewrite it, submit it, sell it and publish it, the small window that was open will be long gone.
The story you should write is the one you are the most passionate about, of course, but be realistic about it. If you love love love paranormal romances, go for it but understand that the marketplace is glutted with them and yours will have to stand out significantly to be noticed. On the other hand, you may want to tell a very small personal story; sure, go for that one, too, but recognize those literary dramas don't usually garner very big advances and may be better suited for a small press.
I spent a year on a novel that I thought was perfect for the market: big themes, big action, big drama. No heart. No love. No bit of my soul in any of the pages. That was my fault. I wrote for everyone else but me. I incorporated everyone else's ideas and hoped they'd make my book bigger. Now, as I push that book aside and consider my next writing project, I have to ask myself the hard questions that I asked you. I have 2 ideas. I love both. One is lighter weight, fun, but has a positive message and a semi-sad ending. The other is more complex, historical, yet personal. Which do I write?
Last time I did this, I solicited advice from both my agent and manager, submitted a detailed outline which they approved and then got to work. The book failed miserably. This time I go with my gut. What moves me? What will be the story I fall in love with and have to tell? Then regardless of its "bigness" or "complexity," it will have been written with passion and I'll be happy with it no matter what happens to it.