Star Wars isn't the only universe that started out as a tight little trilogy and then expanded to include more installments and added adventures. Plenty of authors start out intending to write just three books... and then wind up with a sprawling 12-book series, with no end in sight.
Here are some examples of longer book series that were originally planned to be trilogies.
Top image: Dune, Artist Unknown
Why do trilogies become series? Sometimes, it's just audience demand, or the publisher's desire to keep a popular story going. But also, in a lot of these cases, the authors set out to create universes before they start writing the actual books, and they wind up with grand mythical realms. This is likely why these epics lend themselves so well to other mediums like TV, film and videogames – they are immersive, due to the sheer scope of their universes.
George R.R. Martin, for example, reportedly wanted to create a Tolkienesque world before penning A Game of Thrones. He was no doubt busy creating ancient blood feuds, lineages, and mythologies for his great project, all of which had to be touched on in the actual story. Tolkien sat on a perpetually expanding legendarium — poems, fictional languages and beastiaries — for decades, before incorporating middle-earth into a longer narrative. Upon becoming successful, he was therefore able to draw from these many existing middle-earth elements to write LOTR. Frank Herbert's process was similar. He collected and researched Dune's elements for years before publishing the first book. When it took off, the world of Dune already existed; he simply had to direct it into a narrative form.
So here are some authors whose trilogies became much more lengthy:
Jacqueline Carey published Kushiel's Dart in 2001, the first book in a series taking place in a fictional medieval world, Terre D'Ange. The idea of a nation founded by fallen angels came from the Biblical Genesis and from Jewish folklore. Therefore, like many on this list, Carey imagined a lush fictional world before coming up with a plot — and indeed, the storylines and characters are essentially vessels for exploring this world over several generations (Phèdre nó Delaunay in the the first trilogy, Imriel de la Courcel nó Montreve in the second, and Moirin of the Maghuin Dhonn Carey in the third.
George R.R. Martin was already an established Nebula and Hugo-winning author by the time he took it upon himself to create the Seven Kingdoms of Westeros. He began writing the basis of A Game of Thrones in 1991, adding backstory details over time, and started writing the story proper in 1994. From the very start, Martin was attempting to write a saga of Tolkienesque proportions, and knew he would require at least three novels to accomplish his vivid endeavor. Martin pitched this trilogy to to his agent — but soon realized the scale of universe he had created required additional books, and conceptualized another three books. He claims that A Song is two linked trilogies, and yet one continuous story. The series, however, seems to continue expanding, with Martin writing prequel novellas and planning to conclude the series definitely with an eighth novel.
The SF masterpiece that is Dune famously grew from an article Frank Herbert was writing, entitled "They Stopped the Moving Sands." After researching and writing for half a decade, Herbert had the manuscript successfully published in two parts by Analog, due to its being uncommonly long for the standards of the time. After countless rejections, Herbert got his break through an publisher primarily known for its auto-repair manuals. His landmark effort earned him Hugo and Nebula awards. Despite Dune's critical success, Hebert could not yet write fulltime, and serialized Dune Messiah though Galaxy magazine. By the early 1970s, Herbert could afford to wholly devote his attention to fiction, and he finished out the trilogy with Children of Dune. And at least for a while, he was referring to "the Dune trilogy" in interviews, as a finished story. Later, though, he added God Emperor of Dune, Heretics of Dune, and Chapterhouse: Dune. Frank's son, Brian, has since expanded the universe into countless sequels and prequels, even describing the events of the Bulterian Jihad.
Based on a 1977 short story that appeared in Analog Science Fiction and Fact, Scott Orson Card published the Nebula and Hugo-winning novel, Ender's Game in 1985. Card has since explained that the novelized version's main purpose was to set the stage for Speaker for the Dead, an unfinished sequel at the time, and its own sequels. These would follow a repentant Ender seeking atonement for exterminating an entire alien race in the first novel's Bugger War Xenocide. The fourth book, Children of the Mind, was itself originally written as the latter half of Xenocide, and would have effectively completed an original trilogy. In addition to the main timeline, Card would eventually go on to create the "Bean Quartet", beginning with Ender's Shadow, a parallel cycle that follows the series' events through the point of Bean, Ender's hyper-intelligent companion from the first book., essentially allowing him to rewrite the entire saga all over again.
Often considered one of the most important works of Science Fiction, Isaac Asimov's Foundation trilogy humbly began as eight stories published in Astounding magazine between 1942 and 1950. These tales, which were inspired by Edward Gibbon's The History of the Decline and Fall of the Roman Empire, were lightly reworked into the novels Foundation (1951), Foundation and Empire (1952), and Second Foundation (1953). By the 1950s, Asimov had turned his attention to popular scientific nonfiction, of which, in typically Asimovian fashion, he produced an inconceivable number of works. Foundation's Edge in 1982 would mark the prolific author's return to SF, after an imposing cash offer and decades of supplication from fans and publishers alike. Asimov then penned Foundation and Earth, another sequel, the prequels Prelude to Foundation and Forward the Foundation. In doing so, Asimov tied in the Foundation universe with his Robot and Galactic Empire series, effectively creating an impressive unifying mythos, which is continued to this day by other authors.
Douglas Adams' multi-format franchise originally began as a 1979 BBC Radio comedy broadcast, and eventually evolved into a six-book "trilogy": The Hitchhiker's Guide to the Galaxy (1979), The Restaurant at the End of the Universe (1980), Life, the Universe and Everything (1982), So Long, and Thanks for All the Fish (1984), and Mostly Harmless (1992). Naturally, the franchise was labeled as a proper trilogy on the third book's publication — but was then dubbed "a trilogy in four parts" upon the fourth's release. Taking a jab to the rampant SF-trilogy-practice, Mostly Harmless' first edition cover actually features the legend "The fifth book in the increasingly inaccurately named Hitchhiker's Trilogy". In addition to writing the books, Adams had supervised the series over a multitude of mediums, including desgining a Hitchhiker video game, contributing screenplays to a TV adaptation, and had some level of creative input in stage productions. Despite Adams' passing in 2001, the franchise continues to grow, with Eoin Colfer having written a sixth book.
Le Guin actually wrote a couple of stories set in Earthsea a few years before she crafted her first novel there, A Wizard of Earthsea. She followed this up with two more novels, The Tombs of Atuan in 1971 and The Farthest Shore in 1972, which seemed to conclude the Earthsea saga pretty neatly, including a strong conclusion to Ged's saga in particular. But then, as time went on, she seemed unsatisfied, particularly with the way women were represented in the original novels. So she went back and wrote a fourth novel, 1990's Tehanu: The Last Book of Earthsea, with a much stronger focus on the female characters. This turned out not to be the "last book of Earthsea" after all, because she followed it up with a story collection and one more novel, 2001's The Other Wind.