Hard Mode Filler

So, you've finally beaten World 4 of your 8-World game! You're halfway done!

Wait... why does World 5 look exactly like World 1? And why are all the enemies replaced with harder, faster ones? And the time limit's smaller, too! And, what the heck, your HP Cap is lower?? What's going on here?!

Welcome to the Hard Mode Filler. Unlike a standard difficulty setting, this is when a game forces you to replay something earlier at a harder difficulty during the same playthrough. It also counts if the game doesn't actually give you an ending after beating an easier difficulty, instead just unlocking a harder difficulty which the end of the game lies behind. Sometimes it shows you interesting new dimensions behind older levels, while other times it is simply a case of the developers being lazy and not wanting to design more levels.

Compare Second Quest, where this happens to the entire game after the first playthrough is over. See also Where It All Began, when you have to return to an earlier area as part of the story. Sometimes a Boss Rush works the same way, but you're only redoing the bosses.

Examples of Hard Mode Filler include:

Super Mario Bros. - While you didn't have to replay the entire game at a harder difficulty, five of the later levels were exactly the same as earlier levels except with harder enemies, smaller platforms, and sometimes Bullet Bills flying through the air. Oh, and more and/or longer fire bars.
- You could also "Press B to select a world" at the end of the game, and go back and play with the Goombas all replaced with Buzzy Beetles.
- In the original FDS version of Super Mario Bros.: The Lost Levels, you have to play through the first eight worlds several times to reach Worlds A through D, the true final stages. And even some of those are just harder versions of existing levels.
- Super Mario Bros 3 did this with the Koopaling bosses. The later worlds had the same type of boss battles, except that the ground would shake and stun Mario whenever the boss would jump. Also counts for the Boom Boom battles, which were all the same, save for extra obstacles thrown into the room, and sometimes he would get either wings or steroids to jump around the room wildly.
- Super Mario World did this with the bosses, too: each of the first three had a harder version. And there were also a total of three Smashing Mallet Traps of Doom rooms that were identical in layout, except the second one was cluttered with spinning saw blades and the third one had lava pits.
Most of the Prankster Comets in Super Mario Galaxy just make a previous mission harder for a new Star. Speedy Comets give you a time limit and take away checkpoints, Fast Foe Comets make hazards move faster (thankfully you're usually only forced to redo a segment of a mission), and Daredevil Comets make you redo a boss, or even a whole mission, with one hit point (turning Bouldergeist from a relatively easy foe to That One Boss). However, the Cosmic Comets (a race against a shadowy doppelganger) and Purple Comets (a coin hunt) can still be pretty unique.
- Super Mario Galaxy 2 has 120 Green Stars after you've defeated the game which are hidden throughout the levels and require the player to make progressively harder jumps to reach them...oh, and the last star is a real pain.
Boulder Dash for the NES did not show you the credits until you beat four progressively harder versions of all six worlds in the game, increasing diamond quotas, decreasing time limits, and changing some parts of the level. Getting to World 7, you would find that it's just a harder version of World 1.
Punch-Out!! Wii—Title Defense mode. You have to refight all the characters from the game with remixed movesets, and you have less HP (Some enemies could even cause you to lose with a single KO).
- Not to mention Mike Tyson's Punch-Out!! for the NES, which made you re-fight Piston Honda, Bald Bull and Don Flamenco a second time in tactically riced up form with halved HP.
A lot of Shoot'Em Up games make you replay at a harder difficulty after you finish it once, a feature called "loop" by fans.
The entire Ghosts N Goblins series is infamous for this. Most of the games in the series forces the player to play through the entire game twice in order to fight the final boss and see the ending. The second time the player play through the game, they're forced to seek a special weapon required to defeat the final boss.
Devil May Cry 4 does this when you switch characters.
Metroid Prime: Hunters did it with bosses. Aside from the final bosses, there were only two bosses in the game and you had to fight them both four different times, each time with harder difficulty.
Star Prince and the first two Robot Ninja Haggleman games do this in Retro Game Challenge, as does Muteki Ken Kung Fu in the sequel, although there's a code to begin the game from the second playthrough, similar to Zelda 1.
Rhythm Heaven's "boss" stages are remixes of older games glued together into one harder game. After the 60% mark, even normal levels are just variations of older levels. The remixes and variations are pretty different, but the marketing of "50 minigames" is questionable. For the record, the game has 24 minigames, 10 remixes, 16 hard versions of minigames, 6 endless games, 8 guitar songs, and a few toys.
Rare's GoldenEye has two Surface missions. The 2nd is at night and That One Level to some fans.
Theme Park boiled down to this. After the first few missions, every level boiled down to the same thing, but suddenly the customers spent less money, inflation was much higher, and everyone began hating you
Since the levels in the original Populous were generated with an algorithm, they often fell into this (and Schizophrenic Difficulty) -- you'd invariably get levels resembling early ones but harder. Such filler levels were usually bunched together, though you could skip past them if you earned a good enough score.
Bit.Trip Void was very guilty of this during the songs Ego and Superego, repeating lots of the beat patterns from earlier songs but throwing in some extra white beats to make it harder.
Some levels of Lemmings were repeats of earlier levels with differences in time limit, skill set, and other things. The most famous is the repeat of "We All Fall Down" on each difficulty, adding more Lemmings each time. In some cases this presents the opportunity that an efficient strategy you used on an early level could be reused later, eliminating whatever challenge may have been intended.
Flash game This Is the Only Level has a variant of this trope. There are 30 stages with the same level except there is a catch every stage, like inverted controls, barriers, and creative ways to unlock the gate to the 'next' level.
- And then there's the unlockable FML mode in the second game which involves random spikes and random changes of the level layout that will almost always result in your immediate demise. "FML" officially stands for Frustratingly Manipulative Level, but rest assured that you will be spewing the more common kind of FMLs as you try to beat it.
1942's second half is a rehash the first half with a higher difficulty.
In Twin Cobra, stages 6 through 10 are the same as 1 through 5 with different, much harder enemies and bosses. And then the whole game loops indefinitely after that, without so much as a "congratulations".
Crisis Core is also guilty of this, with a lot of the later harder missions taking place in what looks like the exact same dungeon as the easy missions, except you have more ground to cover, and the Palette Swap enemies get more deadly.
In Astro Boy Omega Factor, Astro fails to save the world after defeating Pluto and the world's strongest robots. However, the Phoenix shows up and grants Astro Time Travel so he can restart his adventure and Set Right What Once Went Wrong. For some reason, everything deals double damage now, and enemies have better reaction times and more hitpoints. Replaying through this unlocks the real final stage and the Golden Ending.
Mega Man Battle Network 4 is a particularly infamous example of this. After beating the main story once, loading a save file prompts the player to start a New Game+ on a higher difficulty level where all their chips are carried over. Idiotically enough, though, both a shortcut-unlocking item, and the Navi Customizer both have to be gained three times. No wonder it's considered the worst in the series. Oh, and then there's the post-game area, Black Earth, which, due to Bass being unfightable until you get all 6 souls, which in turn requires aforementioned 3 playthroughs, has to be done on the highest difficulty setting.
The developers of Guild Wars, once realizing that everyone and their grandmother had finished the 3 campaigns and ran out of things to do, introduced a "new" dimension to the game to encourage players to actually play the same missions and areas with faster and harder monsters, and they literally called it hard mode.
- Although as a trope mini-exception, they did increase the time limit to obtain rewards for timed missions while in hard mode.
- And then to make hard mode, well, easier, they introduced more PvE skills in the expansion, Eye of the North, that only players can use to defeat the bigger and badder monsters.
Some earlier games on the Sega Master System, like Safari Hunt, Ghost House, or My Hero, only had three levels, and basically followed this trope to the letter.
The Challenge Chambers in Portal invoke this trope, taking several of the prior testchambers and making changes to increase difficulty, such as reducing the number of Weighted Cubes you have available, or making certain walls un-portal-able.
To see the true ending of Arkista's Ring, you must play through all 25 levels five times; the enemies become extremely fast and tough in the later loops, especially the ninjas. After the first loop, you receive the titular ring, which restores HP with every step.
In World of Warcraft, "Heroic" dungeons are earlier dungeons tuned to a difficulty for a moderately well equipped player at the current level cap. For the most part, the enemies hits harder, drop better loot, etc. However, some of them add new boss abilities, or an additional boss.
- Cataclysm has brought this one step further by introducing heroic versions of popular content from previous expansions, as well as the standard "Hard Mode" that it has been used for previously. The fanbase seems to be split between newer players who are excited to see this hyped content for the first time, older players are happy to replay the "golden age" content, and less enthusiastic players who think Blizzard is trying to exploit fanservice to pump out cheap content.
  - Somewhat justified as they wanted to get update content to better reflect the passage of time. Going through the new VC and SFK is jarring at first, but in many cases more fun. Now, if they could fix the other dungeons the same way.
Rift uses the same method as World of Warcraft with expert dungeons, which are exactly the same thing in idea and execution.
The arcade version of Commando has eight semi-unique levels, but has No Ending and starts over after you "complete" it, while the NES version, which has only four unique level designs, has a A Winner Is You ending after 16 levels (four loops).
Black and White has an interesting example with world 4. It is world 1, but supremely messed up by Nemesis. You have to destroy three well protected stones which cause fireballs to assault your village regularly, lightning to strike randomly, and constant rain.
Legend of Kage has you play through the levels in four seasons, with increasing difficulty each season. After that, you start back in spring.
A variation occurs in Zuma, which will give you a new color ball and increase the speed by repeating the same three worlds over and over again. For the first three repetitions, an extra level per world might seem to avert this, but it's played completely straight between the third and fourth repitition.
The Raiden games restart at a higher difficulty, and IV has a True Final Boss at the end of the second loop.
In King of the Monsters, you fight the five other monsters and a clone of your player character... and after beating him, you do it all over again except the enemies have more health and the locations are slightly swapped.
In the original Rolling Thunder, the latter half of the game consists mainly of harder versions of the first five stages, with additional traps and enemies not in the early stages. Only Area 9, the penultimate stage, is completely original and not a rehash of an earlier stage. The arcade version even has an optional stage select feature that allows players to start off at any of the first five stages.
Kirby 64: The Crystal Shards has something that is either a very mild example or a subversion. The first stage of the final planet is more or less a Palette Swap of the very first stage (including the color-coded "hit this barrier with this ability to remove it" being a different color), but any difference in difficulty is negligible at best, turning this into a Breather Level.
The last three levels of Halo: Combat Evolved are repeats of earlier levels, with minor variations added. For example, level 9 is basically level 3 with the Flood and some damage to the ship added.
Warning Forever combines this with Boss Rush and exalt it to a form of art. You see, there is only one boss, the Pure Heart. You defeat it, you fight it again. But it takes notice of how it was defeated previously, making it harder for you to defeat it the same way. The boss learns from you too: if certain weapon was effective in defeating you, the next form will have more of that weapon. The evolved form of Pure Heart is something to behold.
Sonic Colors does this with the bosses; the bosses for Planet Wisp, Aquarium Park and Asteroid Coaster are harder versions of the bosses for Tropical Resort, Sweet Mountain and Starlight Carnival, respectively.
In Mega Man III, after completing the eight initial stages, rather than going directly into Wily's castle like in the previous game, the player is forced to go through redesigned versions of Spark Man's, Needle Man's, Gemini Man's and Shadow Man's stages, only instead of fighting new robot masters, the bosses are all identical-looking "Doc Robots" who use the powers of the Mega Man II robot masters.
Alpiner had eighteen levels, consisting of the same six mountains repeating with faster falling hazards and decreasing time limits.
Munch Man (a Pac-Man clone for the TI-99) had 20 levels. The next 20 were the same maps with faster, tougher enemies. The 20 after that had even faster enemies and invisible walls.
In Tutankham, after the fourth stage, the first four start repeating over and over again with more locked doors.
In The Legend of Zelda Skyward Sword, the Skyview Temple has to be visited twice. In the second visit, more (as well as new) enemies are included, some new puzzles are added, and the boss is a fight against three Stalfos, offering a much trickier fight.
E3M9 "Warrens", the Secret Level in Doom Episode 3 starts out as completely identical to the episode's first level. Until you reach the "end" and hit the "level complete" switch, where the walls around you suddenly come down and you find yourself in a giant room with a Cyber demon. And it only gets more insane from there...