After I wrap up the Season Avatars series, the next big writing project I want to tackle is an urban fantasy trilogy. (I also have a partial draft of a children's book, but so far that's a standalone. I may write a sequel or two later if there's interest.) I have a general sense of the characters and the situation, but instead of plowing in, I'm trying to outline not just the first book, but the overall series. There are still a lot of unresolved questions, but at least I'm thinking not just about what the heroes are doing, but also their antagonists want, what are the worst things I can do to the heroes, and what their emotional arcs should look like. I'm not sure yet if I'm going to write the entire series before editing and releasing it, or if I'll go book-by-book the way I did with the Season Avatars series. As long as I end up with finished books that make sense, the path to them doesn't matter too much.
If you write a series, do you plot it all out before starting or do it book-by-book? Feel free to share in the comments.