I'm currently juggling the intricacies of how each character will grow over time across several plot lines. On top of that I need to take into account how this first storyline will influence a separate but tangential tale. Then both of these stories will combine or more like intertwine as each will need the other to complete the climatic ending of a third story. Okay I realize this is somewhat confusing, so I'll attempt an example that is unrelated to my current piece. Hope you like algebra.
a---> A where character one is little a and through book grows into big A.
x--->X our nemesis also develops and grows stronger in the book so by the end he is big X
So book1 the finale is A battling X where A defeats X and becomes Ax ....
b--->B now little b is character two and he develops in book two and grows to be big B
Ax is still around and helps b become big B so you have B+Ax as a team.
y-->Y is our new book 2 villian and the duo defeat this new villian thus becoming (B+Ax)*Y==>(B+Ax)y
So you start with By and Axy and they are stronger than ever having survived through two installment already.
This time they will be battling z who turns into Giant Z by the end and they of course win the day as always.
So the final equation is then (By+Axy)*Z ==> (By+Axy)z where our heroes defeated Z and made him little z.
But hey this is the final installment so the boss should be huge and heck if we are crafty enough maybe they have been developing over the course of the three books, right? So taking that into account then in book 1 we could have a little z thrown in, who by then end of book 1 is big Z. Then by book 2 big Z has become Giant ZZ. Finally by book three he's beyond imagination growing to a humongous ZZZ that must be whittled down by our characters who have just now mustered up enough strength and wisdom to overcome the bad baddie beast of ZZZ. (getting tired for some reason, but lets continue anyways.)
book 1 = a + x + z ==> Ax +Z
book 2 = Ax + b+ y + Zz ==> Axy +By +ZZ
book 3 = Axy +by +ZZz ==> Axyz + Byz
Phew, are you still with me? (Yes, that weird ass crap up there is really how my brain thinks.) Well then what happens after the big 3 part story ends?.. A new beginning starts of course and with it a bunch of new characters. This time however the setting is changed from what occurred in those first few books and we have so much history to help us create the follow up books. Let's just say for the sake of this discussion that the author kept the 4th book and maybe more in mind when tackling the development of numbers one through three, wouldn't you think that four would be pretty stellar. At the very least it would probably fit and flow better.
I think this is why I now understand why so many fantasy novels end up as epic fantasies. The worlds created by these writers are truly 'green' ever changing and growing as the stories develop over time. In my case, and probably with many other writers, the 'green' state is alive even before the first printed copy of the novel hits the shelves. I cross my fingers that all this hard work up front will pay off in the end and that the world I have designed is intricate and wonderful, a place that readers will enjoy visiting from time to time as new installments are released.
Oh, just for fun... keeping the string theory algebra fun alive a putting book four into the equation then you'd have to add in the elements that might foreshadow the characters or events that could take place in this book in the first three books. We'll call this foreshadowing the variable f.
book 1 = a + x + z +f ==> Ax +Z +f
book 2 = Ax + b+ y + Zz +f ==> Axy +By +ZZ +f
book 3 = Axy +by +ZZz +f ==> Axyz + Byz + f
book 4 = f + c + u ==> Cu ... as in Cu L8ter