People usually say that even if you have a planned series - the first book should be written to work a stand-alone because (in theory) even if you sell the first one, there's no guarantee that you'll sell the sequels, and since you're a new author, you want readers to be satisfied with the end. So, I'd probably opt against ending with a cliffhanger and go more for the "wrap up most of your threads but leave some things conspicuously open" approach. Granted, I'm trying to work through this myself - I'm a bit over halfway through the first draft of my first book - which in theory is the first book of a four-part series with a huge overarching plot with the biggest climax at the end of Book 3. So I'm not exactly in a position to say I've done this because I'm attempting it now. But I have consciously set up the "big bad" in this book as someone who can be thoroughly defeated by the heroine for a satisfying ending - while really what I'm doing is writing the origin story that sets up the team I'm going to need in place to fight something much bigger. (In my case, it takes place at a cable news network, and in the whole series the antagonist is the weight of history itself - not to mention a fascist dictator who is hurtling the world toward World War III) - but the first book is entirely concerned with my reporter protagonist fighting her evil editor to get the flailing news channel back on track. I'm thinking whole-series concerns with a lot of my action - but the actual villain is purposefully someone I can utterly defeat at the end of the first book.
