Well series vs. trilogy. With a trilogy you have a limited amount of time. The same can be said for a five book series, but a bit longer. Harry Potter is the first thing I thought of. I haven't read the books and I know the movies aren't the same thing. The movies are a general idea of the book so stay with me on this. Okay the whole ordeal throughout that whole series was that Harry was the boy who lived, Voldermort is the wizard who should never be name, That is the overall story arc of the series. Each separate book dealt with that story arc, but each one had their own separate arc. In the first it concentrates on Harry finding out about his origins, going to school, and dealing with the Sorcerer's stone. Then there was the Prisoner of Azkabahn (I have no idea how to spell it). The order of the Phoenix etc. With each movie/book it give you one step closer to the final battle as well as dealing with an individual problem(s) that happens to become a part of the over all plot. Hermione and Ron's relationship, Harry's friendship gets strained, deaths, the games, etc. Obviously with each problems resolve and questions answered more questions were formed. What I'm saying here and I'm hoping it makes sense is that each book is separate, but at the same time all of the books are connected. You have to resolve something in one book, but the answers found will lead to new questions and they have to lead to the next book. The problem is figuring out what to resolve and that is not easy to answer.
Now to make more sense of this I'm giving my two cents based on what I'm working on. I have finally started the final draft of my book. It is a fantasy novel where human world and magic world collide, but not with the whole human is really a witch or whatever the humans are humans. Anyways I have no idea exactly how long my series will be or if it will just be a trilogy. I had already decided on the issues with the first two books. The first book is more of an introduction of the world, characters, and situation. The main situation is keeping the world a secret. There are also going to be minor issues. Due to portals opening a human gets trapped there and that is the main issue of the first book. Next issue will involve dragons being stolen from the secret world into the human world. Humans and magical creatures are accidently coming into contact with each other and eventually having to interact for one reason or another. The overall issue will involve the political issues of the secret world etc. I really don't want to reveal too much because I fee like I'm getting off topic.
Basically in order for this to work I will have to leave some plot holes open at the end of the first book. Overall it is a good idea to have a few sub plots because I had read quite a few book series with sub-plots that eventually gotten entwined with the main one. Even when they weren't fully resolved there was still a hint of a possibility of a resolve. I had read a book series a few years ago atleast the first three books. I would have read more if it weren't for the fact that I haven't gotten the time to read long books at the time. I swear that is my reason for not continuing. The third had a cliff hanger and someone above mentioned how a cliff hanger at the end of a book deterred them, but because there wasn't any encouragement to want to find out. This cliff hanger of the book I read had it where a character dies and he was one of the main ones. After his death one of his collegues and girlfriend was talking to his tenant and she said something how he is out there still or it isn't over or something. I have actually forgotten about the book untill now.
A cliff hanger isn't a bad idea because in order for a continuation than there still has to be something to resolve. With an individual book everything has to be resolved.