There are multiple books devoted to the subject "What to write". I imagine many of those tomes meet established sales goals for the "How To"/"DIY" genre, and, as a result, drive the author's names ever higher on various best-sellers lists.
I'm sure those books are full of lovely information and formulaic devices. And, as with everything in life, I'm sure that sometimes they help and sometimes they don't. But, do they answer the question at hand? "What am I going to write?"
I've touched on this subject in the past, but I hope this will inspire you to approach your project anew.
The first clue for what to write is simple:
What do you read?
If your answer is "I don't have time", my response is "Good luck. I hope you find your way out of the jungle." What you read, the subjects that interest you, the style and point of view you find most compelling, will influence your work more than you might readily admit.
Many writers begin as copycats. That's not a bad thing. I'm not speaking of plagiarism. I'm speaking of emulating the style and flow of a favorite author. One thing to keep in mind is this: There is nothing new under the sun.
Every idea has been used. Every relationship has been examined. Every plot twist has been previously devised. What makes each story different is the unique personality of the writer.
Vampires have been a topic for literature since before Bram Stoker. The past decade and longer has witnessed a flourish of storylines, adult and young adult, illustrating the capacity of vampires for love and life. This is direct opposition to the frightening, though somewhat romantic, notion of vampires extending through the advent of Ann Rice, et al.
There are hundreds of "me too" storylines and the market will support the most compelling. The secret is found in discovering what drives your own unique passion.
So again, what do you read?