An Unexpected Gameplay Change is when the play style of a game is altered in the game itself. When applied to a level, it becomes a Gimmick Level. Now take that concept to a global scale.
In some cases, the control scheme changes accordingly to the levels or scenes played, in a way such that the game blends several genres or styles in one. It's not a party game, because the default scheme is that of a different genre. This can also extend to the scale of a game franchise, so the control scheme and entire play style are completely different for each concurrent installment; the first few games might be your classic platformer, but the games after that go from being a first person shooter, to an RTS, to an MMORPG.
That's Gameplay Roulette, in a nutshell; the trend of some games or franchises to switch up their gameplay in a frequent basis. Done well, it will provide a dose of variety and flexibility in order to avoid stagnation. Done badly, it might be perceived by fans as a desperate attempt from creators to try to either keep the present formula fresh or look for new genres that can fit into the series.
A rule of thumb for telling the difference between Unexpected Gameplay Change and Gameplay Roulette: If you can point to one game style that's the "main" genre of that particular game, and the rest are all just deviations from the norm, then it's Unexpected Gameplay Change. If you can't say that any style is the main genre, then it's this trope.
Subtropes are the Party Game and Minigame Game, which feature Gameplay Roulette almost by definition.
The Digimon World franchise definitely qualifies. First: Raise your digimon at the gym and compete in real-time battles, the story is a young boy trying to piece together what has happened to the denizens of File Island. The second is a dungeon crawler, with turn based battles, the story being a young kid becoming a tamer, then fighting off the Evil Organization. The third plays somewhat like a mix between Pokemon and Final Fantasy, the story reflecting this. And the fourth ditches humans altogether and brings back real-time battling and introduces co-operative playing in teams of up to four players.
By this point, the only thing you can be certain about any new mainline Final Fantasy game is that it's some form of RPG. And the spinoffs will probably at least have RPG Elements.
As seen in the AVGN review, the NES/SNES/Game Boy Godzilla games all fall under this trope.
Jak and Daxter moved from a fairly light platformer to a gun-heavy Darker and Edgier pair of platformer/driving games to an all-out racing game by the time of Jak X.
The Sgt. Frog games are, appropriately enough, completely random. For instance, Chou Gekijouban Keroro Gunsou Enshuu da Yo! Zenin Shuugou requires playing through ten different training missions before the final boss battle—and every mission is a completely different style of play from the others.
Grab a copy of LittleBigPlanet 2 and head to the community. Have your mind blown at the variety of gameplay that can be found in nearly 9 MILLION levels. The Controlinator, which didn't appear in the first game, lets players make their own controls for anything in their level, so eventually, people took their favorite genres and applied them to this game.
The Sly Cooper series began as a platforming game with a focus on stealth. With the second game, the developers gave the other members of the Power Trio playable-character status and shoved more combat into the game, throwing in many, many minigames for good measure. The result was a rather awkward stealth/platforming/actioner hybrid. The third game stretches the mixture even further, with the end result that there's an Unexpected Gameplay Change every ten seconds.
Sonic Adventure was almost a literal roulette wheel, with six playable characters — each of whose levels used different styles of gameplay, from "run/platform to the end of the level" through "fishing minigame". Sonic Adventure 2 dropped this to run/blast things/find stuff, which was reused for Sonic the Hedgehog (2006) with Sonic, Shadow, and Silver's gameplay, respectively.
Quadruply for Sonic Adventure. Most people count the six playable characters as separate gameplay styles in and of themselves — but if you pay attention, there are even more beyond that! In Sonic's story alone, you didn't just run to the end in a 3D platformer — you also snowboarded, sandboarded, played pinball and slots, engaged in aerial combat, fought hand to hand (or spindash to spindash), and raced in a hoverkart. Not to mention you can raise artificial life forms here and play whack-a-hole/hedgehog there...
Sonic 2006 brought this Up to Eleven. There were three "main" gameplay styles, with Sonic focusing on running, Silver focusing on exploration/level manipulation, and Shadow having a blend of running and combat, but each character had two "Amigo" characters who play occasionally switched to. Sonic had Tails and Knuckles, Shadow had Rouge and Omega, and Silver had Blaze and Amy.
All of Space Rangers. Normally a simultaneous turn-based space action-rpg, but then sometimes you shift genre into arcade shooter, or into an RTS which you can also turn it into third-person shooter, or a text adventure which might range from choose your own adventure to economic simulator. If you want to avoid those, be a bit careful on getting quests. Don't worry too much, though, all of them are fun.
Star Control is a serviceable action/strategy. Star Control II is an excellent action/adventure. Star Control 3 is a crap version of 2, with some odd colony management stuff thrown in for no discernable reason.
The Super Mario Bros. games tend to be more successful than most in this. They've successfully spun off into RPG, Driving Game, Party Game, and other franchises. Nintendo as a whole seems to love doing this. The secret to their success at not alienating fans is that these shifts are always explicit spin-offs; there's never any worry that they'll make another "normal" entry in a series as well.
Transformers suffers from this trope somewhat, with its myriad attempts to pump out specialized lines for specialized fans: the Alternators and Titanium lines for collectors, Go-Go-Gobots for preschoolers, etc. It has also not had a consistent genre for its video games, spanning from platformers to Fighting Games to Action/Adventures.
Practically every other game of the console Yu-Gi-Oh! games attempts to break from the standard card game mechanic of its real-life counterpart by adding various levels of RPG Elements, or Duel Monsters-inspired board games, none of which have been nearly as successful, especially when the former gets bitched out for changing the cards so drastically in order to fit into the new mechanics that veteran duelists can't fathom why they now work the way they do. The handheld games, on the other hand, stick largely to the card game mechanic... and the fans like it that way.
There's also a few non-video game attempts to branch the Yu-Gi-Oh franchise away from the card game, two of which were featured as popular spin-off games in the anime. None of them caught on.
A third, called Yu-Gi-Oh! Hexors, exists as nothing but a board game that had no mention in the anime or manga. It never even saw a second booster pack release, being cancelled after the starter sets failed to sell.
Yu-Gi-Oh! The Falsebound Kingdom has almost nothing to do with the card game aside from the monsters used being taken from there. The gameplay is a mix of an RTS and an RPG. Long story short, the heroes get sucked into a video game made by KaibaScott Irvine, where they have to fight their way out using computer programs based off of monsters from the card game.
The Army Men series was often criticized by professional reviewers for its schizophrenic gameplay changes. The first two games were isometric strategy/tactic games, most of the others were Third Person Shooters, but there were also a pair of isometric Desert Strike-style flight games (the Air Attack series), a Robotron 2084-style overhead shooter (Green Rogue), a RTS, and even a Tomb Raider ripoff (Portal Runner).
Warlords is a series of turn-based strategy games. The spin-off series Warlords: Battlecry is a series of real-time strategy games.(Although one could easily say RTS/RPG, as the RPG part isn't just tacked-on like Warcraft, it's actually done better than a lot of RPGs). Then comes the spin-off Puzzle Quest series of puzzle-RPG's... including Puzzle Kingdoms, which is basically puzzle/RPG/RTS.
Magic: The Gathering had several unsuccessful attempts to branch out into the video game world until Magic Online, which was basically the game online. Some games, like Battlegrounds and Armageddon, had RTS elements; others, like Tactics, had Turn-Based elements. Consistently, however, the most successful video games in the series are the ones that emulate the real card game the closest (Magic Online, Duels of the Planeswalkers).
The Shining Series was originally a first person dungeon crawler. Then it turned into a Turn-Based Strategy franchise before releasing one more first person game and deciding it wanted to be a action RPG. Even the action RPG games bear little in common with each other. It later turned into a fighting game.
The Dept Heaven series has a different genre with each episode (but not spinoff). The only thing similar between the gameplay of the episodes is that they're all a type of strategy game. Except Riviera: The Promised Land, which is a JRPG.
Alone in the Dark (2008) included third person melee combat, first person shooting, platforming, and driving. All in one game!
Battletoads has many styles of gameplay: beat-em-up vertical (both ways), horizontal with all 4 directions (belt-scrolling 3D) or just 2 (2D), high-speed obstacle courses, a racing level against three rats in succession, and even a (kinda) puzzle level that features snakes that move in insane patterns and you gotta stay on top of them. Sometimes the same level features more than one of these! And YES, this all forms a wonderful, cohesive experience.
The Adventures of Bayou Billy is nominally a side-scrolling beat-em-up, but there are also on-rails shooting sections and driving levels interspersed throughout the game.
Conkers Bad Fur Day for the N64 starts off as a regular platformer, but throughout the game you'll be racing across lava, flying around picking up villagers for Dracula, third-person shooting at Teddie Nazis, and tearing cavemen apart while riding on a dinosaur. The final boss pits you in a robot suit fighting a Xenomorph. The player almost needs to learn a new control scheme for every level.
Gunstar Super Heroes, the GBA Remake/Sequel to Gunstar Heroes, has this in spades. Every level is practically a different game.
Haven Call Of The King had this as its entire marketing gimmick. "No other game has so many genres - it's like getting everything you like in one game!" The developers were so convinced that they had a new blockbuster franchise that they made Haven a trilogy and made the ending of the first game a Cliffhanger. The problem was, since the energy of the programmers was spread out over so many different game styles, no single one really excelled and the gameplay was decidedly average as a result. Call of the King tanked, and the remaining two games were canceled.
The Impossible Quiz. Usually, it's a straight quiz game (accounting for some bizarre logic), but then you have to do things like help Dr. Eggman mutilate Sonic the Hedgehog's corpse, stroke a cat, or clip toenails.
Kirby Super Star includes multiple platformers containing different elements (Spring Breeze is a short remake of Kirby's Dreamland, DynaBlade has a world map, Gourmet Race is a foot race against King Dedede, The Great Cave Offensive is the closest to a Metroidvania the series got until Kirby and the Amazing Mirror, Revenge of Meta Knight has timed gameplay (which Kirby games generally lack), and Milky Way Wishes forces Kirby to gain abilities using methods other than his trademark inhalation) starring the title character, a Boss Rush, and minigames featuring completely different gameplay. The Nintendo DS remake, Kirby Super Star Ultra, adds even more sub-games (Revenge of the King is a remake of the original Kirby's Dreamland's hard mode, Helper to Hero is a Boss Rush featuring the Player Mooks, Meta Knightmare Ultra has you go through most of the original platformers again as Meta Knight, and The TRUE Arena is a Boss Rush featuring the bosses created for the remake), and adds three new minigames (the original minigames, complete with their original graphics, are unlockable).
Mission Critical at first glance appears to be your typical first-person adventure game with regular and timed puzzles. However, halfway through the game, the game switches to a space Real-Time Strategy, of all things, and back again. While YMMV, most reviewers agree that the blend of adventure and RTS was done very well and makes perfect sense in context (the player is the last surviving crewmember of a starship and is forced to fend off occasional attacks from the rival faction by personally commanding combat drones in RTS mode).
The Simpsons: Bart's Nightmare for the SNES is another game that focuses on actively inducing this trope. The hub of the game is a standard - if odd - platformer, but entering the levels let you play as a super hero, a godzilla, Indiana Jones, a miscroscopic being in a blood vein and Bart being chased by Itchy and Scratchy. Each one has completely unique controls and gameplay. There was a sequel called Virtual Bart, but its gameplay and systems were more forced and it didn't do as well.
Interestingly, Bill Williams, the man who designed Bart's Nightmare, created many Gameplay Roulette games for Atari 8-Bit Computers. Alley Cat was about a cat who had to jump into the windows of a house and perform various cat-like tasks such as catching mice, knocking a bird cage off a table and chasing the bird, drinking milk out of saucers guarded by sleepy dogs, and other things. Though the core gameplay didn't really change. However, a more drastic change occured in Necromancer. The first level had you planting trees and protecting them from evil spirits and creatures. The second had you navigating a dungeon with the ability to summon the trees you had planted and protected previously. The third had you engaging in a fight against a wizard of some sort.
Virtual Bart features a literal case of this, as Bart's stages are chosen by virtue of him being strapped to a wheel.
The Super Star Wars games did this too, eg the first game switches between sidescroller and free-roaming 3rd-person Vehicular Combat, then to a first-person rail-shooter in its final stage, similar to the old arcade game.
Incredible Crisis is a sequence of completely different games for each level — except one, which is replayed for three of the four characters. (Arguably, it's the most annoying of all the levels to have to play again.)
As a very minor example, The Unholy War combines features from a Strategy game, but with combat played out as a 3D Fighting Game with limited moveset.
Rygar NES combines overhead and sidescrolling gameplay, ala Zelda 2.
Barbie Super Model, where you switch from being a side scroller where you dodge obstacles, to a matching game, to remembering the correct keys for Barbie's dance moves in a modeling show in the span of a few minutes. However, it was made for children who may have had shorter attention spans and needed the variety of gameplay.
Ultimate Stuntman, an obscure unlicensed NES game by Codemasters/Camerica.
NiGHTS: Journey of Dreams goes from flight stages to puzzle bosses to 3-D style platformers all within the same game.
D2 has a mix of survival-horror style and Myst-style adventure segments, with a controller-based "gun game" combat system, and RPG-like stat advancement.
Nightbreed: The Interactive Movie starts with a driving/stealth section, where you have to drive around a map while both keeping an eye on your fuel gauge and avoiding/smashing through road blocks until you get to a cemetery. Once there, you get chased around by one of the eponymous Nightbreeds; you have to click the mouse in time with the character's arms. After that, you end up back at the driving level, going back to the cemetery. And finally, the game switches to a dungeon crawler, where you wander around identical caverns while looking for five characters to rescue. And then there are all the minigames, from fighting off "Sons of the Free" to dodging laser sights, all of which have their own gameplay mechanics. You have to do all this on one life.
The game experienced a severe degree of backlash because the demo and trailers mostly emphasized the Hack and Slash elements and most players picked it up expecting a Metal Themed God of War only to find the Real-Time Strategy to be the major focus.
Family Guy Video Game has a different genre for each playable character: it's a beat-em-up for Peter, a stealth game for Brian, and a platformer/third-person-shooter for Stewie. There are also short flashback minigames a la WarioWare.
Grand Theft Auto: San Andreas does this every couple of missions. While the GTA series are known for shootouts, drivebys, and wild car chases, San Andreas stuffed every possible idea into the game's main storyline and side quests. This includes, but is not limited to:
Learning how to fly various planes and helicopters, a VTOL jet, and a jet pack (all of which have entirely different control sets) as well as miniature toy cars and planes (though these at least use similar controls to their full-size counterparts)
Incoming! is pretty consistent about making you blow shit up, but how you do it changes constantly in the Story mode. A mission might make you play as a defense turret, an helicopter ferrying supplies around, a tank, a jet, and then go back to the turret again. This also applies to a lesser extent to its spiritual predecessor, Darklight Conflict.
Done deliberately in Asura's Wrath, where the entire gameplay style changes depending on the situation and story.
NieR is usually a third person hack'n'slash RPG, but at times it changes to a Zelda knockoff (complete with Item Get fanfare that's one note away from copyright infringement), a 2D sidescroller, a Bullet Hell rail shooter, and, of all things, a text-based adventure game.
To a certain extent, Space Station Silicon Valley is a game built around this concept. You switch between many different animals with varying gameplay styles over the course of the game. Additionally, the last stage of each of the four environments is effectively a minigame, such as a boxing match or a shooting gallery.
The first two Crash Bandicoot games had a couple of levels where you rode animals that were well received, but generally was mostly a platformer, Crash Bandicoot 3: Warped, on the other hand, has two scuba diving levels, four jet ski levels, four motorbike race levels, two tiger riding levels, and three airplane flying levels, of which the first two are about shooting down targets, and the third is a race. That's fifteen out of thirty two levels. Add in one boss who plays as a bi-plane shooter, and subtract that two of the bonus levels are just alternate entrances, and that's roughly half the game spent not on foot platforming.
Each of the four worlds contains one level that must be played as one of the four characters you unlock: One's a Kangaroo who has ridiculously high jumps and gets a lot of platforming without gliding, one's a bomb dropping flying penguin who gets a lot of aerial assault type levels, one's a strongman yeti who gets a lot of combat heavy sections, and one's a gun wielding monkey who gets a lot of shooting stages.
On top of this, each world contains a speedway where you're flying around either racing a set course, or trying to smash a bunch of items quickly enough. Hidden within these is a Hunter mission, which will have you doing aerial combat.
Each world also contains a top down shooter for Spyro's dragonfly to go through, that needs to be done to unlock the Sparx power-ups.
And then there's all the things in the levels themselves. Amongst other things you'll have to do for 100% completion there's skateboarding, yeti boxing, first person shooter sections, rail shooters, racing, cat hockey, submarine combat, turret shooting and other stages which changed the way the game itself played, such as sections where you had to do an Escort Mission, fight a number of Mooks, make it down a really long slippery slope, a level where your fire breath froze enemies, turbo-charged running sections and a Dual Boss that made use of an inifinite power up to be fought. There's probably others too. The developers all but admitted they had gotten very bored of making Spyro during the second game and decided to all out experiment with this one.
Many European computer games of the 8-bit era (particularly those from the Spanish publisher Dinamic, e.g. After the War, Army Moves, Freddy Hardest) came in multiple parts, each of which would load separately and offer a different style of gameplay, often even with a different-looking title screen and status bar. These disparate parts would be linked together by an Excuse Plot and passwords given in between.
Army Moves (Dinamic) had Stage 1 as a Shoot 'em Up in a jeep driving from left to right, Stages 2-4 as a Shoot 'em Up in a helicopter flying from right to left, and Stages 5-7 (on the second load) switch to foot-soldiering action with a little platforming.
Savage consisted of three levels: a side-scrolling action level, Space Harrier-style running-towards-the-horizon, and a flying exploration level.
Mr Bones on the Saturn had this as its main schtick. Normally levels played as as a free-roaming sidescroller platformer, but occasional puzzle or even Rhythm Game sections would mix things up.
Trauma Team is basically six games in one, but consists mainly of treatment games (which are rather like the other Trauma Center games) and diagnosis and forensics, which are adventure games.
The WarioWare series is built around this trope, throwing rapid-fire 5-second minigames with different controls and gameplay styles at the player (though fortunately the rules are always easy enough to be described in one or two words.)
The second Earthworm Jim game features a lot more variety in gameplay than the first, including an isometric shooter level, a recurring Fire!-esque level revolving around bouncing Peter's many, many puppies across the stage with a giant marshmellow, and especially Villi People, which features a change in character to Sally the Blind Cave Salamander who's actually Jim in a costume and changes gameplay three different times (starting with a maze, then a game show, then a game of Simon with pinball bumpers).
Geist perpetually oscillates through first-person shooter, survival horror, puzzle, and action-adventure. This is justified by the game's premise (spiritual possession), as the gameplay mechanics can change according to the current host character.
The Desolate Room combines overhead exploration and Top-Down shooter, with RPG boss battles. Its sequel, The Desolate Hope, removes the overhead exploration, while adding side scrolling shooter and Metroidvania aspects.