There's another approach that will make or break the whole novel: it's the underlying theme. Without one, a novel may as well fall into oblivion.
Yes, we all know what a romance is, what a thriller is, what a mistery is, but without an underlying theme, a novel risks being forgotten. The underlying theme gives it a purpose, a "raison d'être", makes the novel transcend the story and embed itself in our memory.
Example: Pride and Prejudice. Everyone knows it's a romance, proud Lizzy gets to meet prejudiced Mr Darcy and when it seems all is lost they finally get together and fall in love... Obviously a romance, and a very well written one, right? But how many similar romances are there? Zillions, most of them long lost in oblivion. But why are readers still reading and enjoying this one, 200 years after it was first published?
Because of the underlying theme: Pride and Prejudice is about personal freedom, about the freedom to make your own choices and have your own opinions and ideas, choose your own life. And this underlying theme -- personal freedom -- is still relevant today and resonates with most readers. It does not age.
I usually start by choosing a theme that I'm passionate about. The story (and genre) comes from that theme. All the other "ingredients" can be added on, one by one. The underlying theme gives me a purpose to write the story and keeps everything glued together.
What is your underlying theme?