So, if you're not trying to write to the trends, how do you decide what to write?
That's where the world gets dicey, my friends. See, the problem is a lot of people write to trends, not because they are trendy but because they genuinely like the genre/trope/whatever. So the old adage of write what you love doesn't necessarily work here. Yes, you should write projects you're passionate about, but when the thing you love happens to be vampires/dystopian/whatever-the-hell-else-was-huge-and-then-dead you have to accept that said project may never sell.
And that's okay.
Yes, you read correctly. If you have a passion project, by all means, get it on paper, whether or not it will sell right now. Most trends are cyclical. As long as you're willing to hold onto that project until the next cycle, you're fine. Or if you don't care if it sells, you're also fine. So, yes, write what you love...
Unless writing pays your bills.
No pitchforks, please. Obviously professional authors can still write what they love, but we also have to be more aware of the limitations of that. We also have to be aware of contracts. Sometimes a multi-book series that starts as a passion project doesn't end up that way by book 3, 4, 5...12. But it's still a contract, it's still a book you have to write--whether you want to or not. In that particular case, you kind of have to trick yourself into believing you're writing it for yourself.
Perhaps it's a character you've been dying to get on paper. Or a setting you've always wanted to visit. Or even a catch-phrase you finally have the perfect place to use. Find the thing in the book that is for you and hold onto it for dear life.
Yes, writer friends, write for yourself, even if you're writing for someone else.