What you want is a stand-alone book that feels complete, but has sequel potential. That is, have the bones for a second conflict in place, but resolve all of the threads so that when the person sets down the book they think the book finished everything of import it brought up.
In other words, as a first time author, you want to focus in on the problems of a set of characters and leave the big stuff in the background; you want to make it unimportant.
- The Hobbit vs The Fellowship
- Mistborn: Final Empire vs Mistborn: The Well of Ascension
- Star Wars: A New Hope vs Star Wars: The Empire Strikes Back
- Harry Potter & the Sorcerer's Stone vs any of the others in the series
All of these openers inevitably forshadow a greater conflict, they all make you want more; but, every first in each series stands on its own and could have stood on its own without ever having had a sequel and still been "good". Maybe not amazing, but something you look back on fondly. They all do this by having a primary conflict that can be resolved, on which the book solidly turns. If you do not do this and people get to the end and see: "oh there's going to be a sequel" you're in danger of losing people and getting negative reviews, which you do not want as a first time author. You do not have a relationship of trust, so your first book has to prove to both agents, editors, publishers, readers, and sellers that you know what you're doing, that you can complete a work.
If you leave them wanting more, it should be a reader's wistful hope, not an inevitable promise balanced upon a cliff.
I know, I know, the interludes are damn cool. The Empire Strikes Back, Order of The Phoenix and many other midway stories that end "incomplete" or on a "downer" sound awesome. And you do get to write those eventually; but you don't get to start with them.
Take it from me. I wrote 200,000 words last year and I just put them in my trunk. I sat down with the goal of writing "A roller coaster of a story that when you get back to the station you realize has tricked you and as the book ends you see the sharp turn that is about to take everyone over the cliffside." I literally wrote that down as my goal for my book. Well, 1.5 years later, I have an unsellable manuscript that needs a lot of work that I'm not willing to do because I'm too in love with that roller coaster idea. So now I'm going back and doing what I should have done in a different world.
Write what you need to write emotionally. But if that emotion includes being published, think very, very, very hard about heeding this advice. I don't know that I should have a year and a half ago, but I know I need to moving forward.