There’s a lot that goes into crafting a satisfying ending to a novel. So I’ll take a little longer covering this point than some of the others.
One problem I sometimes see, even in published books, is a new character suddenly introduced near the end for no apparent reason. Any new character who shows up at the end had better be a bit player, or had better have an organic reason for being there. Preferably both.
Let’s say Hero is on a quest to free the Rightful Heir to the Throne from the Forbidden Fortress. If you’ve written the whole story from Hero’s point of view, you have had no opportunity to show the Caretaker who looks after the Heir, and the Hero probably didn’t even know such a person existed. That’s fine. Introduce him at the moment he becomes integral to the story and move on.
But as the happy company leaves the Forbidden Fortress after their Successful Battle Against the Forces of Evil, if they just happen to run into a Merry Minstrel who attaches himself to their group for no reason other than he’s going to be important in Book Two, you have a problem. Either wait until Book Two to introduce him, or bring him in far enough in advance of the Successful Battle for him to matter to the story.
Characters matter, which is why I say characterization is the most important element of fiction. Don’t treat your characters like chess pieces to be moved around at your whim. Treat them like people who have purpose and meaning in their lives. Then the reader will also feel that meaning.