I've heard "write what you know" many times. But what do I really know about Vampires and Demons? About Time Travelers? Honestly, I only know what I've read from other authors.
Let's take Vampires. I am a total fangirl of them and will read almost every book, see every movie and give every TV series a try. I got the idea to write one but really needed to make mine unique. So I took my favorite Vampire books (Jeaniene Frost's Night Huntress series, Kerrilyn Sparks Love at Stake series, Kresley Cole's Immortals After Dark, Lynsay Sands Argenaux series, Bram Stoker's Dracula, Chelesa Quinn Yarbro's St. Germain series) and made a list of the advantages and disadvantages of each. I looked through that list and decided what made sense to me. Then jumbled them all up, put my own twist on them and wrote The Envoy (Available now from Lyrical Press).
Ghost? Well there's more freedom there because there are so many types of Ghost. I watched Ghost Hunters one day they had a marathon. This lovely paranormal researcher and a sexy ghost started taking shape in my head. From that came Ghostly Passions (releasing June 1, 2011 from MuseitHot Publishinhg)
And then I took on Death. While most books whoever is Death is male. But one author, W. Somerset Maugham, wrote a short story "Appointmentin Samarra" in which Death was a beautiful woman. It stuck with me. So I start thinking after rereading Piers Anthony's Incarnations of Immortality series. Oh I didn't want to try to touch that one but the idea of the different incarnations having a physical presence? Oh yeah, I liked that idea. Which led to my current WIP! I'm still not sure of the title but the two in the running are My Lady Death and Death and the Detective. In which I also mix in my love of Romantic Suspense novels. I just need to finish it.
So if you want to write - then write. The ideas are all around let your brain mix the ideas around and you'll be surprised what comes.