Conqueror From the Future

Take Over the World meets Time Travel. This is a character, very common in Comic Books and occasionally appearing in other forms of Science Fiction, who comes from a distant future, where the technology has advanced to the point where time travel is possible. Having found conquering his own time either too difficult or too easy, he travels back in time to the present, where he sets about using his advanced technology to conquer our world and become an Evil Overlord. Rarely does it occur to them that, if they were meant to succeed, history would already have recorded it, or that they might end up screwing themselves up by tampering with time.

Examples of Conqueror From the Future include:

Anime and Manga

In Mahou Sensei Negima, Chao Linshen is a subversion: the major antagonist for a huge portion of the story, bringing advanced technology from the future to meet her goal, she seems like this, but with two key differences: a) she doesn't want to conquer the world, just break the Extra-Strength Masquerade earlier than "scheduled" and thus prevent a tragedy, and b) she's arguably not even a Well-Intentioned Extremist, because she specifically avoids going to moral extremes to avoid becoming one: She doesn't lie, and doesn't kill: her army of robot minions are equipped with disarmament beams.

Comic Books

Marvel Comics has Kang the Conqueror, Rama Tut, the Scarlet Centurion, and Immortus. The catch is that these are all actually the same guy: he's traveled through time so often, and created so many Alternate Timelines, that there is now an entire Legion of Doom called the Council of Kangs made up entirely of his own iterations. Immortus, it seems, is the original Kang, who is now a Boxed Crook: forced to spend eternity undoing the Continuity Snarl that is the Marvel universe thanks largely to him.
- The miniseries Avengers Forever is little more than an attempt to tie together all those threads.
- The Scarlet Centurion also appears in Marvel's alternate-universe Squadron Supreme limited series, albeit without the Continuity Snarl baggage.
Dr. Doom also occasionally plays this role, thanks to his invention of the Time Platform.
In The DCU, the most notable examples are Epoch, the self-proclaimed Lord of Time, who comes from the year 3786 to regularly have his butt handed to him by the JLA, Chronos the Time Thief, a present-day crook who acquires time travel technology for the same purpose, and the Time Trapper, who is from so far in the future that he is one of these to the Legion of Super-Heroes, who live in the 30th century.
- Lesser known but equally badass are JSA antagonists Extant(a two-bit hero turned into a deranged incarnation of Chaos) and Per Degaton(a time-travelling Nazi).
Two of The Flash's most powerful villains are minor examples of this trope. Abra Kadabra, a mad terrorist from the 64th century, uses futuristic technology to pass himself off as an Evil Sorcerer. Professor Zoom, meanwhile, is a criminal from the 25th century who uses future technology to become the Barry Allen Flash's Evil Twin and Arch Enemy.
In the DC One Million story arc, Vandal Savage, an immortal, evil Julius Beethoven Da Vinci who has been alive since 50,000 B.C., manages to do this, when it is revealed that he is still alive in the 853rd century and has hatched a plot to send a deadly cybernetic virus backward in time to change the future.
Also from The DCU, the Sheeda, a race from very far in Earth's future, thrives by plundering earlier civilizations. They succeeded in destroying one now-forgotten predecessor of Camelot but failed to destroy the present thanks to the Seven Soldiers.
The Future, one of the five heads of the Fraternity in Mark Millar's Wanted, is a Conqueror From the Future clearly based on Kang and his crew. Only crossed with Nazis.
Max Bubba in Strontium Dog, who travels back to the end of the eighth century and sets about wrecking the timeline in order to get revenge on the future. It's unclear just how aware he is that he's wrecking the timeline.
The Disney comic "The World Begins And Ends In Duckburg" features a villain from the future who comes and turns off all electricity. (An Aesop follows about not relying on modern technology.)
Futur10n from The Incredibles comics.
Also from the DCU, Xotar the Weapons Master, a criminal from the 120th century who fought the Justice League of America. His first attempt (the JLA's second ever appearance, in The Brave and the Bold #29) actually addressed the history problem: Xotar had found a fragmentary old historical document mentioning that had travelled to 1960 to defeat the Justice League. The story ends with the document being written, revealing that with all the missing portions in place it is an account of his unsuccessful attempt to defeat the Justice League.

Film

In Star Trek: First Contact, the Borg, realizing that they're having a lot of trouble assimilating The Federation, travel back in time to prevent the Federation from being formed (and assimilate the Earth in its more vulnerable past). It initially succeeds, as a new timeline is created where the Earth is populated by Borg drones, but the Enterprise was protected from the changes due to being in the wake of the Borg ship's "temporal vortex". The Enterprise follows them into the past and sets history right.
In Star Trek 2009, Nero was sucked back from the future to get "revenge" on Starfleet for not averting the black hole which took his home planet from him. Of course, Starfleet has no idea what he's talking about.

Literature

In Soon I Will Be Invincible, the minor villain Polgar, mentioned in passing, is a direct Shout-Out to Kang the Conqueror, right down to the hero Blackwolf's erroneous belief that he is actually a future version of Villain Protagonist Dr. Impossible (a reference to the previously held belief that Kang was a future version of Doctor Doom).
Inverted in Robert A. Heinlein's "By His Bootstraps". The main character is brought from the present to the far future and becomes the world's "dictor" (dictator). Apparently the people from the future have become too soft or something.
Happens after a fashion in Harry Turtledove's The Guns of the South: the time-traveling Afrikaners initially just try to change history, but turn their advanced weapons against the Confederate States when the latter, lead by President Robert E. Lee, starts moving to treat blacks fairly rather than keeping them as slaves and treating them as sub-humans, the way the AWB wants them.
This is more or less the plot of the Drakon series by S. M. Stirling.

Live Action TV

Doctor Who is full of these. The two most prominent examples are the Master, who is the Evil Counterpart to the Doctor's time traveling alien, and the Daleks, who are Cyborg aliens. Both want to conquer the universe...in all time periods, past, present, and future.
In Star Trek: Enterprise, the crew of the eponymous ship, who operate in the 22nd century, become embroiled in a "Temporal Cold War" between an entire race of Conquerors From The Future called the Sphere-Builders (who come from the 26th century) and a variety of forces opposed to their plans to change history (mostly from the 29th century).
The villain Tempus from a multi-part episode of Lois and Clark
The villains from year 2500 in The Girl From Tomorrow ponder this, but decide conquering each nation individually would take too much time compared to taking over their One World Order led by a Mega Corp.
Jeffrey Sinclair in Babylon 5 is an heroic inversion, having come from the future to be a defender against conquerors.
As of the Bad Future shown in "Letters of Transit," this appears to be the mission of the Observers in Fringe.

Tabletop Games

This is the goal of the Architects of the Flesh from the 2056 juncture in Feng Shui. Because of the way that the setting works, the Buro are not doing this just to satisfy a megalomaniacal urge, but because changes in previous junctures on the part of their enemies can lead to their downfall in a critical shift, just like what happened to the Four Monarchs.
According to the Dungeons and Dragons book "Lords of Madness," the mindflayer empire was crumbling at about the same time that the universe was ending. So in one last attempt to save themselves, they traveled billions of years into the past, where the games we play take place. And since they conquered the universe once, they'll probably be able to do it again. They even use their time to study "lesser" civilizations, in an effort to improve their own.
- In addition, it is implied that mind flayers are an evolved form of humanity.

Video Games

Chrono Cross: After the events of Chrono Trigger, Dalton, who's already pretty ticked off after Magus stole the queen's favor, and after Crono thwarted his takeover of Surviving Village in Antiquity, lands in the Dimensional Vortex, and he somehow gets into the present, and raises an army in Porre and conquers the entire kingdom of Guardia. Which, by the way, lasted for about 1000 years and is inhabited by player characters Crono, Marle, and Lucca (and the former two are possibly killed, but this is debated)
This is the plan of Spectre in the first Ape Escape, travel back to various points in history (from the dinosaurs onwards) to try and make monkeys (or chimps, or whatever primate they were supposed to be) the dominant species, not humans.
Metal Slug XX has this as its main plot, with a time portal depositing high-tech soldiers and weaponry to aid Morden.

Web Comics

The time colonists from Dresden Codak, who are sort of a Lawyer-Friendly Cameo of the Enterprise crew from Star Trek the Next Generation.

Western Animation

In Jackie Chan Adventures, Shendu tries to ensure his everlasting reign by rewriting the "Book of Ages". However, Jade manages to get a page from the book before it was rewritten (specifically the part that mentions her joining the fight against the forces of evil), so she remembers the "true" history of our world, and thus manages to gather the heroes to take down Shendu and his siblings and restore history.
In the Mega Man cartoon, Vile and Spark Mandrill came to the past to steal some Dr. Light's new Lightanium energy rods, so they could sell them in the future. Unfortunately for them, Mega Man X went back to the past to stop them.
In the old Superfriends cartoon, the Legion of Doom subvert this by attempting to conquer the future... Conqueror(s) from the Past? Bwah-ha-ha!!!
The Legion of Super Heroes cartoon: in a reversal of the basic setup of "31st-century heroes summon 21st-century Superman to help", the second season has a 41st-century Superman clone summon the 31st-century heroes to help against the warlord Imperiex. Both the Superman clone and Imperiex remain in the 31st century (their past) for the rest of the season. (Note that this only applies to the cartoon Imperiex, not the comic character he was based on.)
In the Justice League three-parter "The Savage Time", Vandal Savage sends a laptop computer and a complete history of World War II to his 1930s-era self, so that he may use the advanced technology and foreknowledge of history to take over the Nazi war machine, defeat the Allies, and conquer the world.
Parodied in Dave the Barbarian, where a nerd that works in a zipper factory gains access to a time-travelling zipper and then makes himself ruler of Udragoth through supplying the citizenry with video games.