It’s not unusual for a novel to contain many different subplots. As long as all the loose ends are tied up by the end of the book, that’s fine. But as you add additional plots, consider how they relate to the main one and how they advance the storyline. You don’t want a side plot that doesn’t have anything to do with telling the main story. It’s possible to have too much of a good thing. The plot should never overpower the characters. They should always create their world together.
Characters make the book. A great character can inspire a series; a less than great one can sink your novel. If you want your novel to work, create characters who will make the reader care what happens to them. The people in your novel must be as real as the people you see and deal with every day. It’s impossible to say what makes a character unforgettable. What is it about James Bond that has kept him alive and spying for so long?
What makes us remember Philip Marlowe in Raymond Chandler’s detective novels or Jack Ryan in the thrillers of Tom Clancy? Wouldn’t you love to create characters so real, so intriguing that they will be used in countless books and films? Not everyone achieves it, but it is what every writer is striving for when he creates characters.