Plot Leveling

"Lucas: Let see here... we got time and space, every human emotion, every natural element and nature itself, life and death... what do we go for next? Dawn: Only one place left to go... *points skyward* we go for GOD! RRRIIIIIPPPP! Lucas: Uh wait, we got that one too. Dawn: Oh."

- $00pah NiN10Doh! - Pokémon: The Fifth Generation

The plot is completed. The hero gets his due reward, the girl, and vanquishes the villain with a series of really cool moves. Everybody lives happily ever after.

But wait. Turns out that fans liked the story so much that they want a sequel. But the hero's story is essentially done. He's supposed to be content for the rest of his life with what he got at the end.

The solution: Level up the rewards and dangers the hero faces to add that extra oomph to the sequel while avoiding accusations of plot recycling. Instead of a mere Mafia boss, the Sorting Algorithm of Evil delivers a beady-eyed Diabolical Mastermind to deal with, but the hero can look forward to niftier powers and legacies.

This leveling up can get ridiculous if the series continues for long enough, with the producers being forced to one-up themselves with every succeeding installment. It might even be carried out to the point that the only way left for the hero to become any more omnipowerful is to make him Ascend to a Higher Plane of Existence or depower him.

This trope has a unique relationship with videogames, since Rule of Fun and Excuse Plot often allow more leeway when designing sequels.

So Last Season plus Post Script Season. When it's the same bad guys getting an upgrade, it's a Lensman Arms Race. Closely related to Sequel Escalation.

Compare Changing of the Guard.

Anime and Manga

 * One could argue this is what ultimately crippled Dragon Ball Z. We go from a military commander who wants to be taller, to an evil demon king, to the proud and arrogant prince of Goku's race that can crush Goku and his friends with ease, to the ruler of the galaxies that commanded said prince, to a genetic amalgam created from various good and bad guys including said ruler, and the... what can probably be called abomination Majin Buu, who destroys planets and people for the hell of it.
 * Sailor Moon held very tightly to its Plot Leveling with nearly every season's Big Bad hunting for some Victim of the Week's trinket. The concept was really stretched in the last season's mangling of the manga story, where the trinkets are known to belong to any of the superpowered senshi—leading one to wonder why the Big Bad never targets any of them until near the very end.
 * One Piece arguably comes with Plot Leveling built in. We start in the East Blue, the weakest of the four main seas. Then move up to the Grand Line, which lives up to its hype of being difficult to sail and survive. Finally, the sailors who travel across the Grand Line's second half, the New World, refer to the first half as "Paradise" out of comparison to the New World.
 * However the author throws a curveball once in a while. For example, right after the Alabasta arc, which Luffy took two defeats to just barely win on his third try against Crocodile, was the Jaya arc, the main antogonist was Bellamy. It took Luffy one punch to put him down, without stretching.
 * Bleach was this until a point where it went out of control, the author decided to remove Ichigo's original Game Breaker powers and gave him new ones which require him to start leveling up again.
 * For all of 5 minutes until he got stronger verions of his old powers, letting him beat someone who was apparantly as strong as him pre-sacrifice without Bankai or his Hollow Mask.

Film

 * It seems possible that the writers of Pirates of the Caribbean: Curse of the Black Pearl were unprepared for sequels. The heroes went from battling undead pirates and local guardsmen to facing Davy Jones and the East India Company. Luckily, there was enough Canon Fodder and references to Noodle Incidents (namely the E.I.C.) that sequel material wasn't too difficult to come up with.
 * The Back to The Future trilogy. The final scene with the DeLorean flying out to rescue Marty's son was a joke scene, and wasn't intended to be taken seriously. High box office earnings and strong positive reaction, however, allowed the creators to follow through with more films. Furthermore, Marty's future is better insured by the end of the third film, as well as Doc Brown, who gets a wife, kids and a hovering time-traveling train.
 * National Treasure. The main characters went from committing the one large crime of stealing the Declaration of Independence, to breaking and entering into almost every famous government building ever built. Then, they kidnapped the president. The writers were unprepared for a sequel, they had absolutely no plans for one. In fact, they changed the ending to avoid a Sequel Hook.
 * The original Highlander movie suffered from this. The mantra, "There can be only one," seems like a joke when you consider the numerous sequels that dart back and forth through the timeline.
 * In The Matrix, Neo eventually becomes The One, and can transcend the code of the Matrix. He no longer needs to actually fight the Agents, because he can simply rip apart their code. In the second film, however, Neo is back to fistfighting the Agents, with the Hand Wave that they've received "upgrades." Smith is also turned into a virus with self-replicating powers. Neo's implied godlike powers at the end of the first film are mostly limited in the sequels to what he already showed off: flying and stopping bullets.

Literature

 * This seems to be the Modus Operandi for the Star Wars Expanded Universe. Can't think of a plot? Introduce another seemingly forgotten Sith faction from nowhere and build them up to be the greatest threat the galaxy has ever known. Done to such an extreme that the most recent main storyline series had Luke facing off against an Eldritch Abomination that was made out to be stronger than the Emperor. For the record, the Emperor has been used as a benchmark for enemy power levels.
 * Done to extremes in the Skylark series. The main hero and villain are geniuses at the start. And their brains are enhanced with each new book until they're capable of understanding five-dimensional physics and building spaceships with their brains. In case the earth getting destroyed wasn't a big enough threat, by the end of the series the whole universe is at stake. Instead of basic science-y weapons, they grab a team of psychic witches to translocate all the planets inhabited by the villainous race to a galaxy that is set ablaze and burns so fiercely that it'll take millennia to cool down.
 * It is characteristic of E. E. "Doc" Smith - he does it in his other works too, including the Trope Namer for Lensman Arms Race. If he starts a book with 1 km long spaceships fighting, by the end of the book he'll have hundreds of 10 km long spaceships fighting. Unfortunately, he kept writing sequels, so each new one starts at the level the previous book stopped at.
 * Terry Pratchett's "Witch" books in the Discworld series were accused of this. Granny Weatherwax always had to fight a stronger foe - in this case, stronger meaning "better at mind magic" - until, as of Carpe Jugulum, the fight didn't seem like it had a point. Terry wisely took the criticism and moved Granny to a supporting role in the Tiffany books afterwards.
 * Done by David Weber in Honor Harrington; Honor gets roughly one promotion per book, and there's roughly one revolutionary advance in military technology per book. So while the big space battle of the first book is one outgunned cruiser versus a disguised battlecruiser in a peacetime skirmish, the most recent books involve battles between hundreds of ships flinging tens of thousands of nuclear missiles at each other in a galactic-level, multi-sided war. Honor has literally reached highest rank possible in both of the navies in which she currently serves. Weber intended to break the cycle by killing her off and letting her children pick up where she left off about two books back, but co-author Eric Flint gave her a literal new lease on life by pushing the newest war up about 20 years and shifting the focus to other groups of characters to keep from having to promote Honor so damn much.
 * After starting as a low-level CIA analyst in The Hunt for Red October, Tom Clancy's main character Jack Ryan has nowhere else to go after serving as President of the United States for two and a half terms and eventually his son and other young protagonists must take over for him as the focus characters.

Live-Action TV

 * Stargate falls victim to this, but usually manages to make the upgrade interesting.
 * They started by defeating a single Goa'uld (Ra) who had a single ship in The Movie.
 * Then, in the the series, it turns out that was just one of many Goa'uld with many ships. However, he turned out to have been the leader of all the Goa'uld, and actually commanded many ships, although they hadn't been seen. But now he's been replaced by Apophis, who's very similar in most respects.
 * They defeat Apophis, and he gets replaced by Sokar, whose schtick is that he was the inspiration for the devil. Yes, that devil. He also has some fancier tricks up his sleeve, and a bigger fleet, that make him a bigger threat to Earth than Apophis was.
 * Then Apophis turns out to not have been defeated after all, kills Sokar, takes over his fleet and his armies, and now he's stronger than ever and out for revenge on Earth, rather than just generically desiring conquest.
 * At some point in here, Osiris shows up as a recurring villain, mentioned as so evil the other Goa'uld got together and Sealed Evil in a Can before the protagonists inadvertently free him.
 * Then Apophis gets killed (for good!) by the Replicators, who threaten to implacably devour all matter in the universe and who have been giving Earth's Asgard allies trouble since early seasons.
 * That threat isn't even dealt with before Anubis, half-Ascended Goa'uld with advanced Ancient knowledge and the ability to conquer all the other Goa'uld with ease, appears. He comes up with an army of unstoppable super-soldiers, which take several episodes before the protagonists even figure out how to kill one.
 * After Anubis and the Replicators are all defeated at once, and the protagonists have acquired ships that can destroy Goa'uld Ha'taks with ease, in come the Ori toilets battlecruisers that can destroy the Tau'ri ships with ease.
 * The spinoffs not running for as long, don't suffer quite as much from this problem. The Wraith remain a constant threat from the first episode of Stargate Atlantis to the last, though they get the occasional upgrade (like the ZPM-powered Hive Ship in the finale), and the protagonists also have to deal with a fancier, more powerful kind of Replicators along the line.
 * But now in Stargate Universe, the Ha'taks have caught up, and can kick the asses of the ships that could destroy the ships that can destroy the space toilets. When viewers politely asked "WTF?", the writers responded by saying that the rest of the universe hasn't stayed stagnant as our heroes have grown stronger. Presumably the Lucien Alliance stumbled across and reverse-engineered some lost bit of Ancient technology; that seems to be the source of most technological advancement in the Stargate Verse.
 * Chuck used this when, after his father rid him of the Intersect Mark I, Chuck intentionally downloads the Intersect Mark II into his brain which comes with Suddenly Always Knew That powers in addition to intelligence information.
 * Buffy the Vampire Slayer went through this in the first five seasons. First season: Vampire (the master). Second season: Three vampires, all especially vicious (Spike, Dru & Angelus). Third season: A "true" demon (the Mayor). Fourth season: A Demonic cyborg (Adam) and a military installation (the Initiative). Fifth season: A god (Glory). Sixth season broke with the formula: its Big Bad  may have been at about the same power level as Glory by that point, but the seventh season ramps it up again with The First Evil and an army of uber-vamps. Eighth season (in the comic books) has the Flying Brick Twilight, who may outdo   and Glory just with his level of invulnerability.
 * Doctor Who has been doing this, most obviously since the new series started in 2005. Game show controlling Daleks, Cybermen and Daleks invading Earth, The Master seizing control of Earth and decimating the population, Daleks again, this time attempting to destroy the universe, Time Lords returning and attempting to end time... then,.
 * The sixth season finale features
 * Supernatural starts with the boys hunting monsters and ghosts, which leads to them hunting demons, which are the strongest foes they face for a while. As the demons they face keep getting more and more powerful, eventually angels enter the mix, and somewhat surprisingly, they're not all friendly. Of course, Satan is a bit of a pain in the ass later, and most recently, this has all been taken to its logical conclusion with the newest Big Bad being somewhere around Death and God's level in terms of power.

Newspaper Comics

 * Arguably this is what destroyed the original Dick Tracy comic series. While in the 30s Tracy would investigate bank robbers and gangsters using magnifying lenses and fingerprint kits, by the 40s he was stopping Nazi supervillains with his 2-way wrist radio and electronic tracking gadgets. This leveling of threat and technology continued for years; in the 50s he stopped a disembodied voice from taking over the world with an atomic laser, and by the 60s was fighting space aliens on the moon in his antigravity space cruiser. In the 70s the strip was rebooted, with Tracy returning to being a cop investigating criminals, but by that time the strip had lost all social relevance.

Video Games

 * The Pokémon games get accused of this with legendary Pokémon. It is true that their affect on the plot got greater in each generation but Statistically Speaking, generation four's Arceus is the only one to have a greater stat total than generation one's Mewtwo. On the other hand, the sheer number of legendary Pokemon introduced kept going up until Generation five, which did not escalate the number...yet.
 * Sierra dropped the ball with this one and their Quest for Glory series. After the first one, where you save a small Barony, you jump immediately to saving the entire world from an evil genie. There's not much they can go from there, so you then save the world from an evil demon, followed by saving the world from an evil vampire, and finally saving the world from an evil dragon. Each one plays itself up like it's somehow worse than the one before it, even though the end results are pretty much the same.
 * Sierra did try to avoid this though. Originally the Hero was going to essentially go from the second game into what became the fourth game. They inserted the third game when they realized the going from foiling a wizard's plot to summon an evil genie to defeating an eldritch horror was a bit of a jump.
 * The Metal Gear series of games went to increasingly absurd lengths (naturally) to justify Solid Snake's continual returns from retirement. Metal Gear 2: Solid Snake involved a replacement for petroleum and theft of nearly every nuclear weapon in the world. Metal Gear Solid involved genetically engineered super soldiers, a clone brother who took the Cain and Abel trope too much to heart, and invisible nukes. Metal Gear Solid 2: Sons of Liberty involved mass-produced Metal Gears, an anti-Metal Gear, a third clone brother, a kidnapped president, a Metal Gear fortress, and whatever the hell happened at the end. Metal Gear Solid 4: Guns of the Patriots had to work hard to top that, but it did. And it was awesome.
 * To give some perspective, Metal Gear Solid 3: Snake Eater, a prequel to the rest of the series, revolved around... destroying a 1st generation superweapon and assassinating a defector. Plus there was something about getting ahold of enought money to take over the world in there...
 * Bowser, of Super Mario Bros., has gone from locking a Princess in a castle to cursing her whole castle with creepy dreamworld-doors, to lifting it up into space, to trying to take over galaxies. But, of course, Mario wasn't lazy either. He went from "my only move is to jump on your head" to literal kick-boxer to using the power of the stars. Oh, the number of his power-up-items also increased rapidly.
 * And the RPGs. Take Paper Mario. First you had invincible Bowser take over the kingdom, then the Shadow Queen nearly take over the world (and it's an unsealed thousand year old demon with lightning powers too), and then Count Bleck (and Dimentio) try to destroy the entirety of existance (in the former's case for good, latter's case to remake in own image). Mario & Luigi series too to an extent, the first game had Cackletta steal Peach's voice, second had aliens try to conquer the planet and mass destroy the population and the latter kinda went down a level again with Fawful and the Dark Star.

Western Animation

 * Though the creators are doing their best to avoid the trope, Kim Possible's uber-powerful, all-purpose battle suit, introduced at the Grand Finale, became so all-powerful (it was even powered up during the first episode of the new season) that the creators had to Hand Wave the non-use of this battle suit as being in repair for 4/5ths of the fourth season.