Structuring it is not a big problem as it's a continuation of the first (three). We think we know the players and their goals, but they will surprise us.
Keeping the Story on the rails is a course in self discipline for me. I tend to wander. Characters and plot take over and want to have their say.
This time, I'll acknowledge that growth and likely wind up writing two or three books at once. One of them (mostly completed) reaches back in time before the first. Who doesn't like some juicy back story?
Intentional lines get us from one point to another. The trick is to make the trip as entertaining as the destination.