If you want to have a thrilling happy ending in a science fiction movie, blow up the bad guys.
There's more to Independence Day than aliens and pilots. There are three different man-woman love stories, a father-son love story, a mother-son love story, a father and mother-daughter love story, a man-and his children love story, a siblings love story, and even a human-dog love story. And all of these different stories are shown with a minimum of fuss and sentimentality. The stories are shown with simple interactions and simple dialog. I'm not sure the words "I love you" are ever spoken. The relationships are shown so clearly that the words aren't needed.
I was lucky enough, once, to see the extended version of the movie on one of the premium channels. It had a few extra scenes not included in the theatrical release. If you're interested, the extra dialogs can be found here: http://www.movie-censorship.com/report.php?ID=587558 Some of those show that Russell Case 's (Randy Quaid) youngest step son, Toby, actually has a severe and chronic disease that requires constant medication. When the alien is brought into the Area 51 building, Case storms in, yelling demands that the doctors there help his son. He won't take no for an answer. It's too bad this was cut from the main showing as it gives Quaid's character more depth and shows a dedication to his family that is lacking in the "official" version.
Sorry, I got a little off the main topic. Movies are much more likely to have sequels than books because of the...(wait for it)...money! Stallone's Rocky films and his First Blood (Rambo) films are all examples of a first movie being successful enough to inspire subsequent stories about the same character. Usually the sequel is inferior to the original, but not always. The Karate Kid had three sequels but none came close to the original. The Die Hard sequels were decent but not as good as the original. Raiders of the Lost Arc (Indiana Jones) was the same deal, although the third one, Indiana Jones and the Last Crusade was about as good, primarily due to the presence and performance of Sean Connery. But the second Jones movie, The Temple of Doom, was actually a prequel. Clint Eastwood's second Philo Beddoe movie, Any Which Way You Can is much better than the original, Every Which Way But Loose. The plot is much more solid, the actors are much better, the resolution is more satisfying, and the soundtrack is outstanding.
Sometimes the lines can get blurred, like The Godfather II, which has some of the movie as prequel and other parts as sequel. In writing, The Lord of the Rings is a sequel to The Hobbit, but in movie reality, The Hobbit has been made after The Lord of the Rings, so it is presented as a prequel. One other point: TLotR is not a series, even though it is referred to as such. Tolkien wrote it as a single book, but due to its length and the cost of printing, it had to be presented as three separate volumes. The later novel, The Silmarillion, is a true prequel.
My own novel, Just Lucky, is another example. Due to the length of almost 200,000 words, I split it into two volumes: Just Lucky: Friends and Enemies and Just Lucky: Love and Hate. If they sell enough and are popular enough to warrant a third volume, Just Lucky: Inlaws and Outlaws (tentative title) would indeed be sort of sequel, although the time line would include stories that chronologically fall before, after, and during the total chronology of the first two. That's an if and when. More if than when, I fear.
A series is a planned number of books that follow characters in a chronological order (or at least chronologically conscious order) to achieve a plot that is too vast to be told in one or two books. Stephen King's Dark Tower books—seven very large novels—are a great example. With a planned series the author is under no obligation to end each novel with a nice tidy ending. He or she can leave you hanging because more is coming. A good fantasy series is Anthony Wedgeworth's Altered Creatures novels. Each novel does sort of finish the small subplot business it's concerned with but leaves big questions unanswered because we know those answers are still coming.
Another fantasy series is The Mida by Kimberly Sigafus and Lyle Ernst. The first two of a planned eight-book series are published and the third is due out very soon. This series involves an all-powerful time-traveling carnival that pops in and out of various times and places that give one of the carnival members a chance to resolve unfinished business from their past. Each novel resolves that specific problem, but also adds a new situation to the overall condition that will carry over to the next stop.
To get to my own writing again, my novel Prophecy of Honor implies a sequel. It almost demands one. But I had no such sequel in mind when I wrote it. I just needed a way to wrap up the story. I have a good idea on how to start a sequel, and some of the situation that will fulfill the conditions implied in the original, but until I have at least some idea of the ending I won't start that project.
If anybody that has read Prophecy of Honor has any ideas, please put them on the Contact Page. If it leads to a published novel I'll include your name in an acknowledgements page.
My other fantasy novel, a much longer one titled Witchery, will be published this Fall. There have already been inquiries from my beta reader and others that know a little bit about it if there's going to be a sequel. This one is in the same boat. I haven't planned one, but I have ideas. But, again, I need at least an idea of the ending before I start the project.
A little more about these various forms of continuation next time. And please, read something!