The opinions herein are my own and should be taken not as advice but as a singular perspective on the industry based on five years as an active author seeking publication.
Let me first get this out of the way: I believe you should write what you want to.
Do not let current bestsellers, book trends, writers, agents, editors, or publishers tell you what to write. In other words, if you love vampires, ghosts, literary style, or police procedurals but fear it’s not sellable, write the book anyway. Write what you’re passionate about and worry about the positioning of the manuscript later, when you’ve actually finished it.
My reasoning behind this is that something that is "out" now could be "in" later, and because books take so long (at least one year after the actual book deal, usually longer – and this doesn’t even include snagging an agent first) to be acquired, made, marketed, and released, by the time your manuscript is published, it could be perfectly on-trend and positioned for success.
The tricky part about “writing” for the pub market comes after you have a full draft. This is the time you need to step back and consider how to best query it to agents. But don’t get too wrapped up in trying to please the agents on your list! That is actually how I believe most writers make a critical error.
Let me pose it as a question: Would you rather get a great agent or a great book deal?
The answer is simple: Your goal is to hold your book baby in your hand. While the agent will help you get there, you can help yourself by identifying how/where your manuscript can become an actual book – which is in acquisitions, not with an agent.
Put another way, when an agent is considering your query, their primary goal is to sell the book. They may adore your work and story but not feel like they’re able to sell it. Agents have editors/publishers in mind above all else – even ahead of the quality of your writing and manuscript. Your best chance to find an agent, then, is to actually write as if you’re going to take your book and query directly to a publisher.
So what is a publisher looking for?
A unique voice: The key to success is establishing a unique and clear voice that tells a compelling story. Voice is a really fuzzy concept, but it is honed over time (aka takes practice!) and is evident on every page.
Bigger Audience = Greater Potential for $$$: This will likely make you a little sad, but your first book will need to reach as wide an audience as possible. That doesn’t mean you can’t write a specific story, but the way you handle it should be careful not to turn too many people off or break too many boundaries.
Stay in a traditional publishing lane: Best not to rock the boat your first time out. Keep your ambitions low but your contributions high. You can still tell a deeply meaningful and powerful story within your chosen genre. Get your foot in the door first by maintaining and respecting the publishing status quo (regardless of whether you agree with it or not).
This is all to say that your task -- after you write the book and before you query it -- is to find a way to break out in enough ways to stand out in a crowded market. Writing for a wide audience will help, and one way to ensure this is to stay in a lane audiences are familiar with. Thus, honing your unique voice is essential.
Practical steps to take if you want to keep the pub market in mind as you write, revise, and query:
Complete a full manuscript. Most of you have done this. Share it with betas and CPs. Be open to criticism and change.
In revision, consider NOT the agent but the acquisitions editor and dream publisher for your book. Where are your favorite books published? Who edited it? Work backward to develop a clear sensibility for what that editor likes. Write to that person's tastes.
Edit and polish your book by honing the voice. Make your manuscript sing! You may be sick of your book by now, but comb through it a few more times for snags that may irk agents/editors. Follow conventions for things like dialogue and sentence structure.
You can’t control the publishing market, and it’s slow to change. On top of that, the process of finding representation, going on submission, and producing a book are even slower. However, this gives you the flexibility and opportunity to write what you want and feel most passionate about, unhindered by the market. Only when you’ve completed a full draft will it be time to revise your manuscript and adjust your query with an eye for publication.