I have the same problem. My current WIP is a horror, and I am devoting a great deal of time to it (revision, revision, revision), but there are two more books to go before it is finished. The thing is, in addition to this horror story (BTW, I don't agree with the tag, but since it's about vampires it's automatically lumped into horror, apparently), I have another story percolating inside the empty coffe pot that is my mind. The grounds for this second story, which I suppose falls into the "sci-fi/fantasy" genre, are set. The filter is in place, the pot is where it needs to be, I just have to pour the water and turn the pot on.
I have actually written the entire prologue in my head. It's killing me to wait to begin it, but I want to finish this first story before I start on something that is going to be altogether different.