I've been pondering this question for a few weeks. Originally, my plan was simply to write for the sheer joy of writing. When I started making a little money, I veered slightly in the direction of trying to write stories I thought might be commercial. I like those stories less than all of the others, even the early ones where I had no clue what I was doing.
Someone recently suggested that I focus on writing a series in a particular genre, because that's where the money is.
That comment brought me up short, for two reasons. One, I don't like to write books that I wouldn't read. I'm not a fan of series books. I read Jean Auel's Earth's Children and I'm hooked on Diana Gabaldon's Outlander series, but I think those are the only two series I have ever started were I read more than two of the books. For me, the a huge joy in reading is encountering new and interesting characters. I thought first Harry Potter book was splendid, with delightful new vocabulary and fascinating characters on every page. I read the second one, and quit.
Having tried it, I find that I do not like series writing, and I do not see myself writing a whole lot of books about one character. (Although I do want to have Ray Bailey meet Bev Deller again sometime, somewhere, somehow....) I can't stick to one genre either. I like to write stories about whatever the character's life involves. It may be romance or mystery or straight up literary fiction. I tried fantasy and I'm pretty proud of Merlin's Daughters. I'm not burning to do it again; however, I do not rule it out because I want to allow myself the freedom to write in that genre if I encounter a character who needs to travel to other realms for some reason.
The second thing that made me pause, was the fact that my heart immediately responded vehemently: "But I'm not doing this for the money!" Which caused me to ask myself why I shifted to writing stories I though had more commercial value. My answer is that at the time I thought it was worth a shot. I really like Bev and I may use her again, but that will only happen if my heart calls me to do so.
I don't want to limit myself in any way as to my choice of genre, subject matter or character. I want to write about what my soul needs to say. If that means I don't make a lot of money, so be it.
The paradox, I believe, is that if I let my heart write the stories, they'll be better. Consequently, they may be potentially more appealing to readers. Money, if there is to be any, can only be an after-the-fact by-product of the process of writing for me. My focus must remain on letting my Muse control what I write.
(Fortunately, I have a day job that pays the bills.)