This is a very scary topic for me to write about, because even though I've written 3 books of SIREN SUICIDES, technically, it's 1 book broken into 3. It has 1 major ending, and it didn't come to me until Draft 4, despite the fact that I planned it out beforehand. As I kept rewriting, the ending kept changing, until it felt right. Now, with ROSEHEAD, my second novel, the story is completely different. I didn't meticulously plot it like I did SIREN SUICIDES. As of this moment, I'm about 90% done, with 31 chapters of Draft 1 completed, at about 92K words and 4 or 5 more chapters to go, meaning that I'm smack in the middle of this wonderful topic on how to close a novel. And I have no idea how it will end! Scary, eh? I think so far from my experience (please bear in mind that it's been only 1 year since I started writing full time) is to let your story close itself. Meaning, keep writing until you can't write anymore. Here is what I mean by it.
If ending falls short in 1st draft, it will expand in the next drafts. Don't freak out if at 30K words you suddenly can't write anymore because it seems like your story has simply dried out. It happened to me with SIREN SUICIDES, at one of my earlier attempts, and I thought I would never return to it, shelving it. But the story kept living on in my head and wouldn't let me go. In short, I have gone through about 4 completely different endings until I found the right one. So, rule number one, remember, your story is like an accordion, as you write it, it will expand, then shrink, then expand, it will keep doing it until it can't do it anymore. When it will stop, you should stop writing it, and whatever ending you end up with, will be your end. If you don't like it, please don't try fixing it. Instead, be done with this book and move on to the next one, because you will see the ending forming in your head quicker than your first time writing, guaranteed.
A novel is like a bundle of ropes woven into a braid. It doesn't matter in what genre you're writing, your novel will have multiple plots simply by the virtue of having at least 2 characters, because without conflict there is not story, and there is no conflict without at least 2 characters, even if one of them is the inner representation of the other. Each of the characters will want something, or something will happen to them, or they will mention some secret about something. Whatever it is, my method is to write it down on sticky notes and pepper my writing desk with them, to remember what loops I opened, to be able to close them at the very end. So I imagine they are ropes strung through the fabric of my novel, and I need to touch upon them as I go, more so on the bigger ones, less on the smaller ones, but I need to close every single loop, or finish the braid, at the end of my novel. What inevitably happens in multiple drafts is that some "ropes" fall off and new ones develop, so that by the end I don't remember what started where. If you feel the same, it's okay! That's what multiple drafts are for, for polishing to tucking those loose ropes into the braid so it looks nice and snug. It's even okay if you don't close all of them, just read 1Q84 by Haruki Murakami and you'll see what I mean.
Each character must reach her or his goal in the end. I touched upon this a little befote in describing plots, but this is a slightly different thing called a character arc (or, at least, I think I remember reading about it called like that, so don't kill me if I'm wrong). In the simplest sense of the word, every character wants something and by the end of the novel it will either get it or not, therefore causing the character to undergo a major change. This is why we read stories, we want to see how a hero overcomes a monster, to be able to believe that it's possible and do it in real life. This means that if by the end of your novel not all of your characters have gotten their goals, your reader will be disappointed and not very happy with the book. We like closed loops, we don't like feeling confused, we like seeing the hero succeed and the monster defeated. Keep writing until you resolve each character's wish, even if it means writing a very very long draft. Remember, it's like an accordion, in the next draft you will shrink it, cutting out the water and keeping solid stuff.
Above all, you must feel you're done with it. It all comes down to you. You're the writer, the creator, and you will know in your gut when your story is over. As soon as you feel it, stop. It might be an arbitrary moment, nothing spectacular, but if feels right, it must be the end. Here is the trick. If you were truthful to yourself, if you really opened up your guts and spilled your deepest emotions on paper, the ending will not only feel right to you, it will feel right to your reader, because the reader connects with you emotionally, and there will be nothing worse if you force the ending because you read in some book that it's how it's supposed to be. Even this blog that I wrote, take note of it but still listen to yourself. Disregard what I wrote here, feel. Does it feel right? Then your story is over. Is there more? Then write more. The only rule you can apply here is this: KEEP WRITING. The more books you will write, the faster you will read your inner signals at how to properly close your novel.
Whew. I hope it made sense to you and was helpful. I'm still working out this for myself, like I said, being in the middle of closing my 2nd novel, so feel free to chime in the comments and let me know what you think!