So here we are, day one, and with a pretty interesting question!
As a general rule, I don’t usually wish for a series I’ve read to go on longer. The thought may pop into my head at first after I’ve just finished a fantastic one (e.g. Harry Potter, The Hunger Games, or the Inheritance Cycle) but in the end I find I am usually happy with the way the author wraps everything up, and I am always wary of the danger of stretching out a series for the sake of the money or for the fans when there isn’t enough interesting material to work with. So instead I’m going to opt for: A book series you wish would just end already!
As soon as I saw this question, there was a particular series that came to mind: