The strange tendency of trilogies to expand and see more and more works added to The Verse. Mainly found in books, but may also occur with movies.
SF author Orson Scott Card has suggested that this is the result of Executive Meddling; rather than allow an author to write the books he wants to, publishers pressure him into producing sequel after sequel in order to take advantage of the preexisting fanbase and milk a Cash Cow Franchise bone-dry. (Note he also made the distinction between genuinely expanding a story versus splitting a story up due to excessive length, such as was the case with his Xenocide and Children of the Mind - not to mention Tarantino'sKill Bill).
Two-Part Trilogy is somewhat related, typically the result of a one-part story expanding into a trilogy.
See also Franchise Zombie. Capcom Sequel Stagnation is a related trope for Video Games.
Turnabout Storm is an interesting case. Originally a 4-part series, the episode count increased to five when Part 3 was split into two episodes via Simultaneous Arcs, but with the final part still being called Part 4 — At least until Part 5, the real conclusion, was confirmed via Cliffhanger.
Alien≥ is the definitive ending for the story of Ellen Ripley, who died by simultaneously falling into molten lead and giving birth to a Xenomorph queen. However, Alien: Resurrection brought the character back as a clone who finally made it to Earth. Up until the release of the Alien Legacy box set in 2000, the first three movies were still packaged in one case as the "Alien Trilogy", with the fourth film packed in separately. It's also been released as a "Quadrilogy" and an "Anthology" (to say nothing of the spinoff Alien vs. Predator films or the 2012 pseudo-prequel Prometheus).
The Indiana Jones trilogy was expanded with a 2008 sequel, with a fifth rumored about the adventures of Shia LeBeouf. Justified in that, like Star Wars, was going to be a longer saga originally, with five or six movies. Things just kept getting in the way of development. In the meantime, much material was added to the Expanded Universe.
Live Free or Die Hard and A Good Day to Die Hard. Something of a stretch, since the Die Hard series doesn't really have an overarching storyline anyway.
Spy Kids. However, it's not really a continuation of the first three, since it's centered around a new family (the family of the previously-unmentioned aunt of the original spy kids, to be exact), though the original spy kids, now grown-up, do appear.
The View Askewniverse, which started out as the Jersey Trilogy and became the Askewniverse Chronicles. There are six films released, with Clerks 3 on the way.
Pirates of the Caribbean. However, the fourth film is an entirely new adventure featuring Jack Sparrow, rather than a continuation of the previous films' arc. Rumors are that 5 and 6 will be filmed back to back.
Shrek Forever After, the fourth Shrek film. They promised this'll be their last attempt to squeeze more dollars out of the green, Scottish Cash Cow Franchise. Then they made thePuss in Boots spin-off (which had already been in development for years when Shrek 4 went into production). Originally, five Shrek films were planned, but the franchise's decline in popularity following the negative reception of Shrek The Third seems to be the cause of Dreamworks shortening the saga to only four installments.
The original Star Wars trilogy covers a timespan of four years between the beginning of the first film and the end of Return of the Jedi. In the intervening years, a prequel trilogy, multiple television series, games, books and comics have come out. The timeline of events in the series now spans from over 5,000,000,000 BBY (Before the Battle of Yavin) to 138 ABY (After the Battle of Yavin). Not only that, but there are multiple "eras" (the Knights of the Old Republic franchise, the Expanded Universe sidestories, prequels and one-shots set during the OT, and the "New Republic", "New Jedi Order" and "Legacy" arcs) that have galactic power flip-flopping between the Rebel Alliance and various incarnations of the Empire. And then Disney brought the rights to the franchise and decided to do a whole new trilogy, also declaring all Expanded Universe material non-canon.
Saw was originally intended to be a stand-alone film. Because of the astounding success of that one, they decided to end on the third movie. Obviously, they came back after the third one and just decided to flesh out a story and keep writing until they came up with the perfect ending. They came up with an additional 5 scripts. The trope was slightly subverted when they had to cut down to 4 scripts because of the success of the Paranormal Activity franchise, which would later cause the death of the Saw franchise.
Set to occur in the rebooted franchise, as well—Sony set out release dates for sequels to Amazing Spider-Man up to 4 plus two spin-off movies, before the second film was even released.
George Romero's Night of the Living Dead film series stood as a trilogy for 20 years and became a hallmark of the zombie film genre before receiving a fourth installment in Land of the Dead, which got some great reviews but was viewed by some fans as a disappointment. Two more installments, Diary of the Dead and Survival of the Dead, came out in rapid succession, to very little cultural impact.
Scream 4, though given that this is the Scream series, it took a couple digs at this.
The credits for Violent Shit III: Infantry of Doom read "End of Trilogy". Cue another sequel, Karl the Butcher vs. Axe, eleven years later.
Deadly Dares: Truth or Dare Part IV, made thirteen years after Screaming for Sanity: Truth or Dare 3.
Rambo (2008), which is more informally known as Rambo 4, arrived in theaters a full two decades after the final installment in the original First Blood trilogy that introduced the character to movie audiences. The fourth film isn't really that gratuitous, however, since it allows John Rambo some closure by having him, at the film's very end, finally return to his father's ranch in Arizona, which he has been away from for close to 40 years.
The Bourne Trilogy with its fourth installment, The Bourne Legacy, where Jason Bourne is only named.
Frank L. Baum tried to end the Oz series at the sixth book, stating that there would be no way to ever contact the Land of Oz anymore. That didn't happen, of course, and he was forced to write several more books before handing off to Ruth Plumly Thompson.
The Tales of Alvin Maker: The fourth book of going-on-seven opens with a chapter-length rant from the author that's titled "I Thought I Was Done" and justifies its existence by meandering into and out of setting background.
Enderís Game began as a novella. When he tried to expand it into the novel Speaker for the Dead, he realized it would have a very slow, boring beginning—but if he stretched that beginning out even further, and turned it into a novel of its own, he'd be able to add details and character development and make it more interesting. Then his publisher accidentally wrote out a contract for "the Ender trilogy," and he had to rewrite a planned standalone called Philotes into the third book. Then he realized just how long Philotes was, and split it into two books, Xenocide and Children of the Mind. Killing off the main character nearly ended the series, but then he realized the thousand-year Time Skip between books 1 and 2 allowed for plenty of Interquels. Now the series is up to double digits.
Eoin Colfer (writer of the Artemis Fowl books) has written And Another Thing..., making this a trilogy of six. The book emphasizes the idea of "no endings" throughout, and while it could be a conclusion to the series, it points furtively in the direction of an ongoing story. The book's cover states that it is "Part Six of Three".
The Salmon of Doubt, which contains excerpts from an additional Dirk Gently novel that Adams was developing at the time of his death, includes notes that he was beginning to feel Mostly Harmless was not the appropriate place to end the Hitchhiker's series (it was a bad Creator Breakdown, ending with all Alternate Universe Earths destroyed and and 80% of the main cast dead.), and implies that Salmon would either have been retooled into the sixth part, or a crossover.
Isaac Asimov's Foundation. Several of his more well-known stories are actually one universe, through Canon Welding. He was a tremendously prolific writer, so only a small fraction of his actual work fits into the background and timeline of the Robots/Foundation 'verse, but that fraction includes some of his most popular stories. It should be noted, though, that the familiar trilogy was originally published in eleven installmentsnote Book 1 is the first four stories, plus a framing story; Book 2 is two stories, one of which was published as a two-part serial, and book 3 is also two stories, one of which was published as a three-part serial.
When Bernard Cornwell was inspired by the popularity of the Sharpe television series to write some more novels, he wrote three books set prior to the Peninsular War setting of the existing novels. They were quickly dubbed the prequel trilogy by fans. Then he wrote two more. (Although as the first three concerned Sharpe's adventures in India and the other two dealt with the earlier part of the Napoleonic conflict, they seemed to have been rebranded the India trilogy.) In the end, he only moved on to other projects when he ran out of early 19th century wars for Richard Sharpe to fight in. The quality of writing remained consistently good throughout.
The Dragonlance Chronicles trilogy received a fourth book, Dragons of Summer Flame, which wasn't written until after several other series and standalone works had been made in the same verse. It was written 14 years after the third book, took place 30 years after, and didn't star any of the characters from the originals (though some of them did have supporting roles). Many feel that the only reason it was dubbed part of the Chronicles series was for marketing purposes.
More specifically, to explain and promote a new Dragonlance setting for the Dungeons & Dragons game.
The Darksword "trilogy" by Weis and Hickman consists of four books. The fourth is written in a somewhat different style than the rest (taking place after a Time Skip and being narrated by a new character), but does conclude important plot threads that the third book left dangling.
There is also another book (Darksword Adventures) which is half novella set in the same setting as the trilogy, and half RPG system.
Weis's solo series The Star of the Guardians, whose third book concluded with the main character rightfully crowned king and all the main characters getting what they deserved... Including the Nominal Hero, who faked his death and returned to a life of penitent obscurity in light of his now-dead One True Pairing. Then Weis published a fourth book to clean them up. (And, even better, she then took her version of the Magnificent Seven to Ascended Extra levels by publishing a trilogy about them! They're not really related, and main characters from Guardians rarely appear in Mag Force 7, but, still.)
Dune is an interesting example. Dune was actually conceived as one long book, with the sequels Dune Messiah and Children of Dune fitting directly after the first. Messiah was fleshed out while writing Dune and eventually became its own novel, which due to its expansion then warranted Children to be expanded as well and also became its own book. God Emperor of Dune and the last two in the series, Heretics and Chapterhouse are genuine examples of a trilogy creep, though the fact that the story is now over 10,000 years past in the originals, it's fair to say that they're a trilogy of their own. Before he died, Herbert planned to write a seventh book which would've been the last of a trilogy, with God Emperor serving as a bridge between the first and second trilogies. With this all said, someone "found" notes in a safe/vault/deposit box supposedly with a lot of notes on how the series was supposed to end. What happened? A prequel trilogy leading up to the original Dune novel. Followed by a pre-prequel to the whole series set in the distant past. And finally the closure of the series... followed by more books in the form of interquels(?). The canonicity is up for debate among fans.
The Wheel of Time was supposed to be a trilogy. Eleven books later, the author died. And what was meant to be the twelfth and final book has since expanded to form its own trilogy. At World Con 2008 Tom Doherty of Tor Books finally put this long-standing rumour to rest: it was originally planned to be six books when Robert Jordan proposed the series to him in 1984, before he even started writing the first book. It's also been stated that Jordan planned for it to be three but Doherty made it a six-book deal due to Doherty's knowledge that Jordan always wrote more than he thought he needed in the first place. Jordan's original plan was for Rand to get Callandor at the end of the first book.
Inheritance Cycle also got a fourth book. The page was originally called The Inheritance Trilogy until the announcement. In this case, it's because the fourth book didn't expand the story, the third and fourth book just got too long to release as one. At 866 pages the last book is almost as long as the entire The Lord of the Rings story. The third book, Brisingr, is another 750 pages so splitting it up was a logical choice.
The Earthsea Trilogy, expanded with Tehanu: The Last Book of Earthsea. Then it was renamed to The Earthsea Cycle. The series expanded to six books (and one of those is a short story collection).
The Belgariad was planned to be a trilogy ("Belgarion", "Ce'Nedra" & "Torak"). Eddings explains in The Rivan Codex that due to length and the publishing standards of large book chains at the time, his publisher convinced him to do a pentalogy instead. Then, when he was writing book four, he realised that he was going to have plenty of material left over, and the second pentalogy was planned. And when that wasn't enough, they went and planned a prequel, that in fact saw the light as two... Of two books each. The Elenium, on the other hand, was a trilogy that received another trilogy of sequels.
Piers Anthony lampshades this by marveling at how long the Xanth "trilogy" has become in the afterword to one of the books. He then mentions the possibility of working on more books in the Apprentice Adept series, since that trilogy was "looking a little sparse" at only three books. It eventually reached seven. The first Xanth "trilogy" happens to have 27 books, with the last of these being titled "Cube Route" (and since 3 cubed is 27, this is another of Piers' in/famous puns). In fact, this seems to happen to Anthony a lot. The Cluster trilogy ended up being supplemented with two Interquels. The Incarnations of Immortality series, though never a trilogy, ended up three books longer than originally planned. As for the Apprentice Adept Trilogy - it's now two trilogies and a seventh standalone.
Mickey Zucker Reichert's Renshai trilogy started out as a stand-alone trilogy. Then she wrote a sequel trilogy. And then she wrote a sequel to the sequel trilogy.
Mostly averted by Tad Williams; his novels are about as Doorstopper as they come, but his series last exactly the number of books that he intends them to last. However, To Green Angel Tower (of the Memory, Sorrow, and Thorn trilogy) was split into two books for the paperback edition and at least one translation. This inadvertently happened with another trilogy of his, in that the book became so huge that it had to be split in half. It's not so much "trilogy creep" as "trilogy overflow."
Robert Rankin's Brentford Trilogy has seven books. The Armageddon series, perhaps in reference to this, is called an Octology, despite there only being three books in it.
Stephen King declared in his afterword to The Gunslinger that he foresaw The Dark Tower eventually comprising six to seven novels, spanning 3,000 or so pages, and taking anywhere from 20 to 300 years to write. He made good on the letter of his promise; the finished series is seven books and runs a little over 3,000 pages, but the series itself spilled over into over a dozen of his other novels, which amounts to tens of thousands of pages overall.
In the foreword for Blandings Castle, P. G. Wodehouse jokes about this by referring to the 'saga affliction' that grabs the attentions of unwary authors and forces them to think up more and more ideas for what was intended to be just one story.
John Marsden's Tomorrow, When the War Began series was originally a very tightly written trilogy, that afterwards ended up seven books. There is also a post-war series as well now.
Raymond E. Feist's Riftwar Saga was written as a trilogy (Magician, Silverthorn, A Darkness at Sethanon). The first book was edited for size by the US publisher. Later, for the paperback edition the original content was restored, but the resulting text was split into two books (Magician: Apprentice and Magician: Master). The UK publisher still produces Magician as a single volume.
John Christopher's The Tripods had a prequel added to it about 20 years after the original. This was not a bad story by itself, but some felt it was a poor fit to the classic series.
A Song of Ice and Fire was originally slated to be three books. By the end of book two, the writer realized he couldn't do it in three, and so expanded it to four. Since then, he's expanded that to six, and then splitting the fourth book in two, making it seven. If he can keep it that way, it would be fortuitous, as within the world of the books seven is an important number.
The French translations of large fantasy cycles almost never use the words "trilogie", or "tetralogie", or whatever, even when they were used for the original. This is because the publishers smelled the cash-cow and made it an habit to split each book into two, three, or even four parts. The French version of A Song of Ice and Fire is currently a dodecalogy and this bizarro-version of the Trilogy Creep will eventually require the non-English speaking fans to shell upward of 200 euros just to get the completed series in paperback.
John Scalzi's ''Old Man's War" trilogy has now gained a fourth book, retelling the events of the third book from another character's POV.
Lynn Flewelling's Nightrunner series gained a fourth book years after the first three were originally published, but this wasn't a change of plan: book three had an author's note explicitly stating that "This is not a fantasy trilogy; it's a series that happens at the moment to be three books long".
David Gerrold originally planned for his The War Against the Chtorr series to be a trilogy, before he realized he was going to need more room. It's currently up to four books.
Robert Ludlum's The Bourne Series has a clear ending in book three, as Bourne/Webb finally kills his nemesis Carlos the Jackal. After Ludlum's death Eric van Lustbader continued the series. Interestingly the film adaptations of the books, which bear only the most superficial resemblance to their source material, also provide a definite ending in the third entry. But then they started making Bourne 4.
Anne Bishop's The Black Jewels Trilogy has six books (and a short story/novella) collection. Averted in that the first three books are a coherent trilogy (all with titles Noun of the Noun), followed by a prequel, followed by the collection which has prequel, inter, and epilogue stories, followed by two epilogue novels.
Maximum Ride suffers from a very sad case of this — the first book held up fine as a solo work, and the next two sketched out a decent story arc. The fourth book released was received much less positively in comparison.
Diana Gabaldon's Outlander was originally supposed to be two books, then three, then four, then a double-trilogy. The seventh book came out in Fall 2009. The author never explicitly said to her publishers that it was supposed to be a trilogy— only that she had at least two more books after the first in her, and she ran with that. Several of the Lord John stories exhibit short story creep, since Gabaldon's idea of a short story grew into a publisher's idea of a complete novel.
Harry Turtledove, particularly since it was unexpected: his TL-191 series started with a one-off prequel, How Few Remain, then a trilogy called The Great War. All of which fitted the planned releases that had been "Coming Soon" in the fronts of his novels for years. Then suddenly the one round-up book that would have dealt with events later on, called The Great War: Settling Accounts, grew to seven huge books, the American Empire trilogy and then the Settling Accounts tetralogy. The vast amount of padding and repetition involved in these seven, along with what is broadly considered to be a significant decline in writing quality, has led some to accuse Turtledove of deliberately writing Doorstoppers to put his kids through college.
The Emigrants by Vilhelm Moberg was originally intended as a trilogy, but the third book was split into two. It is even more noticable in the original Swedish titles, where the fourth book has an Odd Name Out (The Emigrants about leaving Sweden, The Immigrants about arriving in America and traveling to where land is handed out, The Settlers about starting up farming in a new country - and The Last Letter Home continues that story). The second book is normally called Unto a Good Land in English.
Scott Westerfeld dedicated "Extras" to "everyone who wrote to me to reveal the secret definition of the word 'trilogy'". Proving Tropes Are Not Bad, "Extras" is essentially a companion piece with some trilogy characters as Special Guests.
Chris Walley's Lamb Among the Stars trilogy is actually an aversion in that when the third book was written and found to be a Doorstopper, Walley went back and edited the first two books into a single book. The new third book ends the series in such a way that it would be incredibly difficult to continue it. This was a case of subverted Executive Meddling, since Tyndale asked him to do it. Subverted in that having three similar-sized books works much better than two small and then two large.
Cassandra Clare's Mortal Instruments books provide a rather outrageous example. First, there was the original trilogy (City of Bones, City of Ashes, and City of Glass). Then, it was announced that Clare was writing a steampunk prequel trilogy set in Victorian London (The Clockwork Angel, The Clockwork Prince, and The Clockwork Princess). Then, a fourth book centered around the Simon character of the first trilogy was revealed to be in the works (City of Fallen Angels). Then Clare decided to add two more books to the Mortal Instruments story (City of Lost Souls and City of Heavenly Fire) while also stating that this new trilogy of MI books would no longer focus solely on Simon but rather on the entire cast. This YA fantasy book series literally tripled in size. Almost humorously, after writing the third book Clare stated in an interview that she liked half-open endings, and deliberately left hers that way, and that she had no intention of continuing it.
The Icemark Chronicles was originally supposed to be a trilogy, but the author has announced he intends on writing a fourth, set before the trilogy.
The first three books of The Last Dragon Chroniclesform a definite trilogy, with a complete story, and a very definite ending. The rest do continue it, but begin a new story arc altogether.
The Secret Histories series was originally planned to be a trilogy, but the series proved so popular that Simon R. Green decided to make it into an ongoing series.
John Ringo's Troy Rising series was originally planned to be a trilogy, but word on the Ringo forum on Baen's Bar is that his Muse is insisting on continuing the series, much to the joy of many of his readers. The current plan is for five books total, unless Ringo's Muse insists on more.
Meg Cabot's Princess Diaries series was planned to be a trilogy, and the first three books do make for a complete story, but Cabot kept writing books until they reached the number of ten, not counting spin-offs.
The "The New Prophecy" arc in Warrior Cats was conceived of as a spin-off trilogy, but turned into a six-book sequel series.
Mattimeo, the third book in the Redwall series, ended on a note that was clearly supposed to be a wrap-up. Trouble is, author Brian Jacques got addicted to the universe he'd been writing in and had written 22 books (plus additional material) by the time he died.
The Lion, The Witch, and the Wardrobe was originally intended to be a single, self-contained story. Then Lewis decided to write a sequel. Then he wrote a third book, and made sure to make it very "final": the characters sail over the edge of the world and find themselves in Aslan's (IE God's) country, the two protagonists remaining from the first book are told that they will not be returning to Narnia, and Aslan reveals to them his full glory. He ended up writing more books, but stopped at seven, deciding it was a good number to end on.
Just before the third book of the Hush, Hush trilogy came out, the author announced that she would be writing a fourth book too.
Sergey Lukyanenko originally just wrote Night Watch as a single novel. Following its success (in Russian-speaking countries), he continud with Day Watch and Twilight Watch with the co-author of Day Watch writing a spin-off featuring none of the main characters. Then followed Final Watch... and New Watch. Many fans agree that the series has long ago jumped the shark. To try to remedy this the author turned the series into a Shared Universe, inviting younger authors and promising strict quality control over what gets published. As of summer 2014 fans' reaction to new collaborations is yet mixed and undecided.
Averted with the Lightbringer Series. The author intended for it to be a trilogy, but purposefully named it "Series" just in case he went over. And indeed, he later started with a fourth.
Brian Lumley's Necroscope series was supposed to be a trilogy, and book three does have a fairly solid Bittersweet Ending, but then he figured it was too much of a downer and wrote two more books which ended the series on another bittersweet ending. This was expanded by another three books (Necroscope: Vampire World) providing another bittersweet ending to the series. Which was then continued in a pair of Interquel works which ended on a flat out Downer Ending just to shake things up. Finally another three books followed in which, not only was another bittersweet ending, but also with Lumley deciding to Torch the Franchise and Run. Which closed out the former trilogy on book thirteen. Except for the novella, short stories, and another Interquel book he later wrote.
S. Andrew Swann's Moreau Series had a fourth book, Fearful Symmetries, added on five years after the original trilogy was completed.
Lois Lowry's The Giver (1993) had two sequels, Gathering Blue (2000) and Messenger (2004), and was fittingly referred to as "the Giver trilogy" for eight years. Eight years later, Son is released, definitively tying up loose ends and making it a quartet.
The Bartimaeus Trilogy: It was a trilogy, then along came the announcement of a prequel. The page is still called "The Bartimaeus Trilogy", even after the release of the prequel. This is reasonably fair, though, as the prequel book is very much a self-contained story and only features two characters seen in the original novels: Bartimaeus himself and his long-time antagonist, Faquarl.
An odd case in the case of the Green Sky Trilogy. The first book set up the story, the second was more or less a Perspective Flip from Teera's POV, the third dealt with the fallout of the events on the first two books. But...Snyder decided to play Death by Newbery Medal on a lead character and realized she shot herself in the foot. Then, a software designer shows up, wanting to make an adaptation of her books. Queue what was perhaps the first video game to be called a canon sequel.
Inverted with The Lord of the Rings: when Tolkien finished it, it was in six 'books', with Tolkien wanting it published either all at once or possibly in two volumes. His publishers split it into a trilogy.
Early editions of The Lightning Thief and The Sea of Monsters had "Percy Jackson & the Olympians Trilogy" written on their spines. There ended up being 5 books in the series.
Philip Pullman's His Dark Materials series was originally a trilogy. It was followed a few years later with two novellas, Lyra's Oxford and Once Upon a Time in the North, and another full novel, The Book of Dust, is expected... eventually (as of this writing, almost twenty years have passed since the publication of the first book).
For the first two books, the The Lost Years of Merlin series by T.A. Barron was advertised as a trilogy. With the third book, it was decided that two more would follow and the billing for the series became an "epic" instead. In the introduction for the third book, the author states that Merlin himself told him that three books would not be enough. This isn't quite so galling, though, when you consider that in the introduction to the first book, he states that the story came to him through Merlin.
When the first novel of Wind And Sparks cycle was published, Alexey Pehov said that the story got too long for a single book, but he'll wrap it up in the second one. Then in the third. It ended up as four novels and two prequel short stories. The irony? It was inspired by The Wheel of Time (but less epic, a bit darker, much snarkier), with a helping of Thief and Garrett, P.I..
Kamen Rider Den-O's third movie was explicitly called "Farewell Kamen Rider Den-O: The Final Countdown", and features a Passing the Torch aspect with the introduction of a new Den-O, and on a meta level it was believed to be the end of the series because star Takeru Satoh was moving on to other roles. Another three / five Den-O moviesnote depending on whether you count the Chō Den-O Trilogy movies as one big one or three smaller ones have come out since then. Possibly lampshaded in the DVD release of "Final Countdown", where a short extra cartoon has one character remark that, for all the talk of "final" and "conclusion", that doesn't stop them from making more sequels.
Played straight with Vapor Trails' "Freeze (Part IV of Fear)." Fear is a "trilogy" that was released in reverse order with Part III, "Witch Hunt," from Moving Pictures first and Part I, "The Enemy Within" from Grace Under Pressure released third.
Though Der Ring des Nibelungen was not originally conceived as a trilogy, it was already four plays by the time Richard Wagner began composing the music, and is not commonly thought of as a trilogy, its official heading is "a stage festival play for three days and one evening before."
Halo is an interesting case, as it was originally planned to be only two games; Microsoft wanted a third game and development time for Halo 2 ran short, so they stretched it out over Halo 2 and Halo 3. Then there's Halo 3: ODST (planned, but expanded) and Halo: Reach (never planned), which roughly fit into the same "trilogy". In 2012, Halo 4 was released, which is the beginning of a second Halo trilogy.
Apollo Justice: Ace Attorney, the fourth game in the Ace Attorney series. Shu Takumi had planned the series to be a trilogy, but Capcom called for another game. The protagonist change was due to the fact that the former three games already told all the stories for Phoenix.
Both "Hitman Trilogy", a boxset for the PS2, and "Hitman HD Trilogy", a similar set for the PS3 and 360, actually feature the second, third and fourth entries in the Hitman series (Silent Assassin, Contracts and Blood Money), skipping the original game in the entirely. The original game in the series, Hitman: Agent 47, was a PC-exclusive; not to mention Contracts is, in effect, a remake of it.
After the release of Leisure Suit Larry 3: Passionate Patti in Pursuit of the Pulsating Pectorals, series creator Al Lowe insisted that there wouldn't be a Larry 4, partly thanks to how Lowe essentially wrote himself into a corner without any idea how Larry 3's ending could lead into a sequel. Instead, he chose to skip the fourth installment (retroactively referred to as Leisure Suit Larry 4: The Missing Floppies) completely and carry on straight into Leisure Suit Larry 5: Passionate Patti Does a Little Undercover Work, where the absence of a fourth game is actually a plot point, and both Larry and Patti have amnesia because the Big Bad stole the Larry 4 disks.
Jak and Daxter was originally intended as a trilogy and thus was written as such: at the end of the third one the Precursors are revealed and the Big Bad is defeated. However, a racing spinoff was released a year later, followed by a PSP game with Daxter in the lead, and a PS2 game developed by a different team than original creator Naughty Dog.
After Another Centurys Episode 3: The Final came out, many people were hoping for a new "A.C.E." game would be released sometime. Low and behold, Another Century's Episode: R for the PlayStation 3. Note, however, that it is more or less a new continuity as opposed to an actual story continuation. Incidentally, Director Terada more or less said, "I don't know why we called A.C.E.3: The Final (due to them apparently having every intention of making a new game). So, let's just say that A.C.E.3 was "The Final PlayStation 2A.C.E. game."
When Fabula Nova Crystallis was conceived, it was originally meant to consist of threeself-containedgames that shared one mythology. Nine years later, and it jumped to six games and about three or four short stories, with possibly more coming along the way.
The Onimusha series was originally conceived as a trilogy, with Onimusha 3: Demon Siege initially advertised as the final installment of the series. It didn't take long for Capcom to produce a fourth game afterward, with Onimusha: Dawn of Dreams (aka Shin Onimusha).
There are rumors that Bioware is working on sequels to the Mass Effect trilogy. The galaxy was saved, so it's unclear what storyline it would follow.