Suddenly Significant City

The elevating of the status of an existing locale beyond its current stature. Cleveland becomes the US Capital, Tokyo becomes the World Capital, Toronto becomes capital of the largest polity in the known galaxy, that sort of thing. The city may of course be very important today, but the transition gives it official political power on a stage above its current influence. At the other end, it can be a sleepy village that suddenly gains much larger stature.

The reasons for this vary. Sometimes, they just aren't given. Other times, other candidates for the position have been destroyed or invaded. Or perhaps they got lucky and economic conditions shifted their way.

Tropes

 * Big Applesauce.
 * Tokyo Is the Center of the Universe.

Anime and Manga

 * After Tokyo gets destroyed, the Japanese capital is moved to Fukuoka in Ghost in the Shell. While a major Japanese city in its own right, it doesn't have any of the fame of Tokyo, Kyoto, or Osaka.
 * In RahXephon. Apparently between 2012 and 2024 North Korea comes out of its shell and the Olympics are held in Pyongyang. Kabul hosts the 2028 games. Oh, and skateboarding is finally an Olympic sport.
 * In the backstory of Legend of Galactic Heroes, Brisbane, Australia became the capital of the United Earth Government which was established after almost of a century of warfare that left much of the world devastated and practically all major cities destroyed.

Film

 * Radiator Springs in Cars. Goes from Dying Town to hot tourist spot after Lightning McQueen moves in.

Comics

 * In Gold Digger, the Diggers family members aren't really weirdness magnets per se, but none the less, after their adventures lots of weird cosmic entities tend to follow them home. Home is Atlanta, Georgia.
 * In the Silver and Bronze ages, the little town of Smallville became world famous for being the home of Superboy. Early Legion of Super-Heroes tales showed Smallville in the 30th century having become a big city.

Literature

 * Chicago becomes the capital of the Solarian League in the Honorverse, the largest (by a couple exponents) human polity in the explored galaxy. Considered a war almost destroyed Earth, it seems probable that all the usual candidates for the job were destroyed.
 * In The Door Into Summer by Robert A. Heinlein, Denver becomes the capital of the US after a nuclear war that leaves most of the East Coast uninhabitable.
 * Parodied in The Twelve Chairs: Ostap Bender proposes to organize an international chess competition in the small town of Vasyuki, which would attract famous chessmasters and revitalize the city. Eventually it would become the center of Europe, the whole world and the universe. Of course the whole scenario is just a plot to swindle some money from the residents.
 * In Kenneth Oppel's Airborn series, Vancouver (or "Lionsgate City") has become the airship center of the world.
 * In Harry Turtledove's In the Presence of Mine Enemies, Omaha, Nebraska is the capital of the U.S. due to the destruction of Washington, DC and a number of other major cities.
 * In the Future Boston sci-fi anthology, aliens somewhat arbitrarily decide that all their interstellar trade with Earth will take place in Boston, Massachusetts. While that would be a big deal by itself, a clever local government official negotiating with her alien counterpart establishes the local port authority as a city agency, rather than state, federal, or international.
 * In Laurell K Hamilton's Anita Blake series St. Louis, Missouri has become the supernatural capital of the United States. What Creator Provincialism?
 * In the Galactic Milieu series, the Remillard family somehow managed to successfully get Concord, NH to be designated as the capital of Earth. No explanation is given as to how they pulled that off, but it probably had something to do with Unifex remembering that this was the case when he first entered the Stable Time Loop that created the Milieu in the first place.
 * In the 1632 series, Grantville, West Virginia, USA becomes the technological center of Europe after Alien Space Bats teleport it to 17th century Germany. Later in the series, Grantville proves impractical for major infrastructure, so the town of Magdeberg, Germany becomes the capital of the new United States of Europe.
 * In Gadsby, Branton Hills' population rises from two thousand to sixty thousand, and it becomes a full-fledged city rather than a town.
 * In World War Z Lhasa is the world's biggest city, and Havana the world's financial centre.

Live Action TV

 * In the 1970s Buck Rogers in the 25th Century TV revival, "New Chicago" was the capital of Earth and its colonies.
 * In Star Trek, San Francisco is the headquarters of Starfleet and Paris is the Capital of the Federation.
 * In Jericho Cheyenne, Wyoming and Columbus, Ohio become the capitals of what remains of the United States. Earlier contenders for the title included Sacramento, California; San Antonio, Texas; Cleveland, Ohio; Albany, New York; and Montgomery, Alabama. Most of the other major cities were destroyed in the series premiere.

Music

 * In Wesley Willis's song I'm the Daddy of Rock And Roll, he informs the listener that "Back in 1991, I used to hit old people with folding chairs. Suddenly, I moved to the north side of Chicago, Illinois in the summer of 1992!"

Religion

 * "'But you, Bethlehem, in the land of Judah, are by no means least among the rulers of Judah; for out of you will come a ruler who will be the shepherd of my people Israel.'" (Matthew 2:6, citing Micah 5:2)
 * Also Nazareth was just a tiny town of probably less than 500 people of doubious reputation until one former local started a movement that grew into two of the worlds largest religions.

Video Games

 * In F-Zero: Maximum Velocity, a discovery of pure titatium had caused Bianca City to change from a small village to replacing Mute City as the center of galactic activity. All this in less than 40 years.

Web Comic

 * In Little Worlds, Halifax (the city in Canada, not in the UK) is treated as though it were a much more urban of a center than it actually is.

Fan Fiction

 * In "The Open Door" by Academia Nut, the town of Nesme in Northern Faerun gets conquered by an interloper deity during the Times of Troubles and turned into the industrialized capital of a new country, the Republic of Noctis. Unfortunately, the concept of equal rights for all sentient beings doesn't mesh well in a Medieval fantasy setting, causing most people to view Noctis as a lawful evil country for giving citizenship to drow, goblins, and orcs so long as said creatures follow the rules.

Real Life

 * Gander, a small town in Newfoundland, became famous in September 11, 2001 when American airspace was shut down due to the terrorist attacks. Gander's airport was a designated emergency landing strip for inbound flights from Europe, but the sheer number of planes was never planned for so the small town was suddenly forced to accommodate thousands of displaced passengers until airspace was reopened.
 * The moderately important city of Byzantium was refounded as Nova Roma by Constantine I in 330 and became the capital of the East Roman Empire under the name Constantinopolis. It was always geographically important (being on a strait), but it wasn't Capital-of-an-Empire important.
 * New York and Philadelphia were the major seats of political power during and immediately after the American Revolution. Washington DC was built because the southern state representatives insisted on a more centrally located capital and a certain marshy site on the Potomac river that no one wanted was available.
 * On July 20, 1847, there was almost nothing between the Rocky Mountains and the Sierra Nevada. On July 21, the first Mormon pioneers entered the Salt Lake Valley. Within four years, it had become the capital of Deseret, a proposed state that stretched from the Rockies to the Pacific coast. Congress cut it down into the Utah Territory, but Salt Lake remained the biggest city in the region for decades until surpassed by such cities as Las Vegas and Phoenix.
 * Las Vegas and Phoenix are examples themselves, having been one-horse podunk towns until gambling became legal in the former and air conditioning became widespread in the latter.
 * Canberra was basically a rural district with a small population until 1908 when, because of politics, it became the capital city of Australia. It's now the 8th largest city in Australia and its own territory.
 * Bonn, A Small Town in Germany. Since the government has moved back to Berlin after the end of the Cold War, it's just another unremarkable small town again.
 * Ottawa was originally a backwater town that only existed because of the construction of Rideau Canal. It was incorporated as a city and given its current name two years before it would become the capital of Canada. It was chosen because it was midway between Toronto and Quebec City and its location made it more defensible from an American attack than the other major Canadian cities.
 * Wellington was originally a minor planned settlement in the centre of New Zealand, with the seat of government in Auckland. That is, until 1865, when it was decided that the capital needed to be in a far more central location.
 * Alexandria, Egypt which became the new center of Greek civilization after Alexander the Great founded it.
 * Tel Aviv was just a minor Jewish community near the mainly Palestinian port of Jaffa. In only a few decades, as more and more Jews came there after being persecuted elsewhere, it became the second largest city in Israel and one of the Mediterranean's economic hubs.
 * Nyenskans was a small Swedish border fort-town. It was located on a bit of territory that Sweden ceded to the rising Russian Empire, and Peter the Great decided to build his new capital on the place. That's how St.Petersburg came into being.
 * Earlier in Russian history, Moscow was a quiet town belonging to the Vladimir-Suzdal principality. The Mongols came, the old fiefdoms deteriorated, and a Manipulative Bastard prince came to rule in Moscow. Then one of his successors kicks some Mongol butt. Suddenly Moscow is becoming more and more important until its last prince reunites the shattered fiefdoms and founds the Muscovite Tsardom.