Merged Reality

One possible ending to stories in which the hero or heroes are trapped in an Alternate Universe is to somehow force the universes to merge or collapse into one single time-line that exhibits (hopefully the best) features of both. A way to Save Both Worlds, if you will.

The current world is headed to nothing short of disaster, so the only choice is to put it back a different way.

For making a world from scratch, see World Building.

Sister Trope to Set Right What Once Went Wrong, which deals with changing events in the past in the hope that things turn out better.

Anime & Manga

 * Parallel Trouble Adventure Dual
 * Yumeria
 * In the manga Mugen Densetsu Takamagahara: Dream Saga, awakening Amaterasu is meant to restore both worlds to their original, unpolluted glory. However, the ending offers the idea that Making a Better World isn't always better:
 * The objective of the villain in Noein is to converge the infinite possible universes into one, which would be free from suffering.
 * In Eleven Eyes, Kakeru comes to the realization late in the game that every one of the chosen ones was drawn from another, slightly different, reality since memories and information don't match up. Everyone worries that when the battle is finished, they won't be able to see each other again.  Everyone ends up staying together in the same world, a composite of all of theirs. They'd become such a tight group of True Companions that it would be too cruel to split them apart.
 * Used mildly in the Shara/Shura arc of Tsubasa Reservoir Chronicle. In this case, the main characters don't even realize they are time traveling, but their actions do result in friendlier interactions in the 'original' world. Not that there aren't consequences....
 * from Mahou Sensei Negima tries to expose the magical population to the Muggles. When confronted and asked why, she claims it's for this trope.
 * Fate's ultimate plan
 * Inverted in Super Dimension Century Orguss. The protagonist's actions have caused a Merged Reality, it's wreaking havoc, and it becomes his and others' job to undo the damage and split all the realities and timelines back up.

Comic Books

 * American Comic Book Example: the climaxes of Crisis On Infinite Earths, Infinite Crisis and Zero Hour. DC Comics seems to be in a habit of doing this every decade or so.
 * Subverted like mad with The DCU character Super***-Prime (he used to be Superboy-Prime until DC lost the rights to the name Superboy, now he's Superman-Prime for no good reason). His motivation is that he wants to bring back his world, the "perfect" world... and he's willing to blow up every other world in existence to do it. Moreover, he has no problem killing anyone who gets in his way (or anyone who doesn't), because he judges them to be "stupid" versions of people who'd exist on his world. Yes, to some extent, he is a superpowered avatar of fanboy rage.
 * Fittingly, he eventually gets reduced to
 * Subverted over and over again in Marvel's Earth X trilogy. Mephisto is continually tempting people to change history for this purpose, when in fact it creates alternate universes--so even if the new universe is better, the old one is still there and just as terrible as before. Meanwhile the Elders of the Universe are trying to put all these alternate universes back together to recreate the original--and never mind that this involves the effective death of trillions of people native to the new universes.
 * A Marvel Comics storyline had the Fantastic Four aiding their enemy-turned-ally Amazon Thundra to merge her Lady Land Alternate Future of "Femizonia" with the misogynistic world of Machus to form a composite future of (theoretical) equality between the sexes. The men and women still fight each other constantly with deadly weapons, but Reed Richards just Handwaves that as the Proud Warrior Race version of Slap Slap Kiss. Naturally, Thundra finds this world boring and seeks out an analogue of Old Femizonia to settle in.
 * The DC and Marvel multiverses briefly merge together into the Amagalm universe in DC vs Marvel. Years later they are merged again(but not creating the Amalgam universe) in the JLA-Avengers crossover.

Film

 * The Back to The Future movies, the first had this happening on a small scale to the protagonist and his family.
 * Subverted in the movie Idiocracy. The main character, after being a Human Popsicle for 500 years, wakes up to find the world has taken a drastic turn for the worse intelligence-wise. He proceeds to look for the time machine that must exist by now to tell the world what awaits them if they do not change.
 * Caesar is all about this in Battle For The Planet Of The Apes, since he knows that his parents came from a world where the world was destroyed in an Earthshattering Kaboom. We never find out if he managed it or not, fans disagree.

Literature

 * Piers Anthony's Apprentice Adept series eventually merges the two worlds into one; as the characters are explicitly paired across both worlds (mirror-universe twin kinda thing), each pair merges into one being, and each pair has to time-share their body. Substantially easier for the heroes than for the villains, since heroes are used to putting the needs of others on par with or ahead of their own.
 * Inverted in Terry Goodkind's Sword of Truth series. In the final book,
 * Diana Wynne Jones' book Witch Week ends this way, with the merging character explaining to the cast that they will all melt quietly into the people they really are in the other world. (Since they're probably going to be burned as witches otherwise, this sounds quite appealing.)
 * It should be noted: 'probably' in context means 'if the world isn't destroyed first,' which is what will happen if their world  isn't merged with the other one.
 * Two different perspectives on this are given in The Pendragon Adventure series. Each Territory has a Turning Point where it can either fall into chaos or continue on peacefully. Bobby always aims for the second option, trying to win over Saint Dane. However, Saint Dane talks of "The Convergence", saying that he's also wanting to make Halla (the entirety of the Territories and the universe) a better place - but the chaos has to come before he can achieve his goal.
 * At the end of The Pilgrims of Rayne, after losing to Saint Dane on Ibara, Bobby forcefully traps himself and Saint Dane on the island territory in an attempt to make it better. This might also count as a Ten Minute Retirement.
 * Done absolutely awesomely in Orson Scott Card's Pastwatch the Redemption of Christopher Columbus. The plot is rather complicated, but suffice it to say it emerges that
 * This is because.
 * Averted in Animorphs. When the kids travel back in time to stop Visser 4 from messing with history, they consider using the Time Matrix to change history for the better. However, they hit the Reset Button by accident and put everything back the way it was (minus one host body for Visser 4).
 * The twist resolution to The protagonist had been openly working to destroy the world all along, but it turned out that wasn't the only way to

Live Action Television

 * Arguably the entire plot for the first season of Heroes, which deals with one person who can paint the future, several people with premonitions of it, and one person who can actually time travel and change it. By the end of the season, the foreseen impending disaster is actually averted, albeit in a less than ideal way.
 * On Sliders, the Big Bad of the last season is trying do, he experiments by first merging people from different universes together.

Video Games
"Exdeath: I will return the world to its original form!
 * Reversed in the computer game The Longest Journey: the idea is to prevent the two worlds from combining, at least under the current uncontrolled conditions.
 * Similarly, the combining of the two worlds in the Super Mario Brothers Movie was a plot of the antagonists to rule them both.
 * Tales of Symphonia ends up with this being the goal of the main characters, although for much of the second half of the game they are under the mistaken impression that they need to do the exact opposite in order to Save Both Worlds.
 * It doesn't quite go as planned, as shown by the sequel/spinoff that came after.
 * In Tales of Eternia, the two main worlds begin colliding. The heroes only manage to fix it just in time by
 * Tales of Innocence revolves around multiple factions looking to get their hands on a MacGuffin for different purposes. The main characters' is to unite Heaven and Earth into one paradise.
 * A variant of the Time Travel version is the goal of Chrono Trigger, and the Alternate Universe version is the goal of its sequel, Chrono Cross.
 * A rare example that's neither: In Persona 2: Innocent Sin, . They take it... but as seen in Eternal Punishment, it doesn't entirely work.
 * Subverted in Final Fantasy V, where this is exactly what Omnicidal Maniac Exdeath wants to happen and the heroes are trying to prevent. As it turns out, the power he's after was sealed by splitting the world in two hundreds of years ago, and he can only release it from the Dimensional Rift by reuniting the worlds (which entails the destruction of the Crystals, so with or without Exdeath actually claiming his prize and wreaking havoc with it, everybody's pretty much screwed).

Bartz: To a world of evil!

Exdeath: Are you even listening to me?"


 * One of the items in the GURPS sourcebook Magic Items III is the Healer of Worlds, a MacGuffin designed specifically for this purpose.
 * Something like this is done in the canonical ending in Valkyrie Profile.
 * The plot of the When They Cry series.
 * In Final Fantasy Crystal Chronicles Ring of Fates, the villains are attempting
 * In Mortal Kombat vs. DC Universe, the Big Bad is Dark Kahn, a merged version of Shao Kahn and Darkseid, who starts merging the realities of Mortal Kombat and the DC Universe, while feeding off the rage of the characters. He's also playing the heroes and villains of each world against each other, which think that the other is an invading army. Eventually, though teamwork, Superman and Raiden manage to defeat Dark Kahn and split him into two. Unfortunately for them, Darkseid and Shao Kahn end up in each other's worlds.

Web Original

 * Lex Luthor tries this in season 1 of I'm a Marvel... And I'm a DC. Not in his universe, but the real universe. His goal was to have comics stop progressing when it was just him and Superman, back when times were simpler and they were important.

Western Animation

 * The entire plotine of the Danny Phantom episode, "The Ultimate Enemy." If you believe that the episode was a time loop, it is a bit subverted that the timeline was fixed first not by the hero, but by Clockwork who triggered the good future when he stopped the Nasty Burger explosion that caused the Bad Future in the first place. The rest was obviously Danny making the right decision.