I have not done much developmental editing of fantasy books, but I have copyedited more than a dozen fantasy books. I prefer to keep my involvement in fantasy books to just copyediting.
If you are in the early stages of writing a fantasy book, I urge you to keep the reader’s needs in mind. You are building a fantasy world derived solely from your imagination, which will be difficult for readers to picture. It helps if you have a small number of significant characters; not dozens. Describe the characters in detail, not just upon introduction but multiple times so that we frequently picture them. Describe each new scene in detail: paint it; don’t just sketch it. Dialog should be interspersed with small actions; e.g., folding one’s arms and smirking. Though fantasy authors tend to write very long novels (100,000 to 200,000 words), readers usually prefer shorter novels (50,000 to 70,000).
“David is fast, friendly and helpful. He has a great eye for detail. Super job.”
—Conan De Moe