Usually, just as I'm close to finishing a draft or edits, I get a new idea. I'm not alone, most writers I know have this problem. The idea is strong and begs to be written. It's exciting, because you realize your brain still does actually work and that your well hasn't dried up like you feared after dealing with the same characters and words.
But it's also frustrating. Because usually we aren't ready to start that book yet. There's a 'to do' list in front of that idea.
The book I recently completed has the possibilty of several consecutive stories all relating to one event. The event has shaped each character differently and I'm excited to find out who's good and who's evil. Very excited.
Except. The last book, and it has to be the last book relating to this event, is already pressing into my brain. I know who the hero is and I think I know who the bad guy is. The story concept is shaping and I find I'm thinking about this book more than the one I should be concentrating on. And no, it can't be the second book. The hero from the last book has to be a part of each book prior to his own, without finding his own resolutions. His position is an integral part of each story. You could say, he's an authority figure who will finally get his happy ever after in the last book.
So, I'll write down everything I'm thinking and file it for when I'm ready. That usually works to take the immediacy away. Usually. I hope it does in this case, because there is quite a line in front of this particular idea.