Do you write for yourself, or an audience?

It depends on how drastic those changes are. In general, mechanical or structural changes, I believe should be made. Content changes should only be made if you agree though. Personally, more than writing to an audience or myself, I write the story--I let it go where it needs to, where it naturally leads. So, if you can live with the changes, by all means do them and sell it. If they make you uncomfortable, then don't. You have to decide where you draw the line. It's your work--don't let others pressure you.

If I were you, I'd try to strike a balance somewhere; think about the changes for a while and see if there's a way you can implement them without losing your vision for the story.

As for the market, at some point you're going to have your stories out there, and then it becomes more about writing for your audience than yourself. If you want to be successful at entertaining people, you're going to have to give them what they want.

Good luck!

If you disagree with a suggested change, no matter who makes it, don't do it. It's your story and you have to be true to that. Making a change you don't believe it usually results in flat work, and that won't sell.

However, I don't think there's anything wrong with making your book more marketable. It is, after all, a product. I don't write something just for the sake of "what sells" but I do consider my ideas with a commercial eye and think about my audience. I want them to be happy with my story, same as I want to be happy with my story.

You might try looking at your agent's suggestions and figuring out why they're suggesting XYZ. Is there a way to revise your work so that it satisfies you, but also makes the book more marketable in the way she's suggesting? Like, adding murdered mermaids won't work, but you can tweak a mystery element or a supernatural element that strengthens a paranormal crime aspect.

Sometimes we can tweak to satisfy an agent/editor but do it in a way they didn't suggest. Not saying you have to of course, but looking at it from a different perspective might help you figure out what you want to do.

I write for myself. But if you're going to be successful, writing for yourself is the same thing as writing for an audience. It means you and that audience share the same taste in stories.

As for writing to specifications from an editor, yes, I will, if it's a book I think I can write well, a book I'll enjoy writing, and if the editor asks me to do so. I won't change the book I am writing radically, but I will write a second book that fits the bill.

But I believe the only suggestion an agent should ever be allowed to make is almost like this one. "I know an editor who is looking for a certain kind of book, and if you think you would enjoy writing such a book, give it a try, and I'll pass it along."

As for vision, the only vision I have for any book is that it be written as well as I can possibly write it, and that it tells the best possible story.

There's a difference between an editor asking for certain changes in a book you've already written and submitted, and in your situation. When an editor asks for specific changes in a book already written, you make the ones you agree with, the ones you think will make the story stronger, and you don't make the changes you disagree with. And good editor understands that he suggests, you decide.

Your situation is different. It's fine for your agent to tell you a particular editor is looking for a particular kind of novel. But I think it's a horrible idea for you to attempt such massive changes in your current novel. This seldom works out. If you want to please that editor, it's best, I think, to write another novel aimed specifically at his needs.

I'd suggest keeping this novel the way you intended it, and writing a second to please that editor. I suspect this would also please your agent. It certainly should.