Overarching, Metastory, Superduper Plot Stuff
No outline, no exercises, just a simple and straightforward statement: You can have an overarching plot, if you want, and frankly I'd recommend that you try to, but it's more important that you have a goal that you are slowly moving towards. I talk a lot about characters making decisions about who they are or who they want to be. If you want to have a powerful series with a strong ending, when you open up the story ask yourself where your character is trying to go, and who they need to become. Then start walking that direction. Slowly. Subtly.
That's . . . that's pretty much it. I mean, I think it is. Good luck.