Some of these are alternate worlds, some are reality altered by time travel, and some are simply imaginary.
Justice League International #59–60 (December 1993–January 1994), Gerard Jones and Will Jacobs.
The barbarians of the 70th Century captured the time machine used by their robot enemies just moments after the Sim escaped into the timestream. Their leader realized the only way to find them was to prevent them from having left, which they had only been able to do with the aid of the JLI. So he traveled into the past, to prevent their foes from gaining their powers.
Test pilot Hal Jordan’s simulator was diverted, and Abin Sur chose Guy Gardner to replace him as Green Lantern. Rex Mason was blocked from entering the Temple of Ra, and continued his archaeology career instead of becoming Metamorpho. Kara of Atlantis remained in enchanted sleep for several more years. Wally West arrived minutes too late to be struck by lightning when meeting his idol, the Flash, eventually becoming a car mechanic.
The four people whose lives had been altered were strangely drawn to each other. Guided by one who remembered the original history, Green Lantern traveled back in time to repair the damage.
Flash Annual 7 (1994), Mark Wheatley and Allan Gross.
In a world in which Kid Flash became a TV superstar, Barry Allen died battling Captain Cold. Shortly afterward, Wally began losing his powers, his feelings of guilt for not being there to save his mentor leading him into psychological paralysis. A decade later, the story follows the production of a movie about the Flash’s life, and the struggle between Wally and Len Snart over whose story will be remembered... until Wally finally finds the courage to tell the truth.
3-issue mini-series (1996), Mark Waid and Fabian Nicieza
A spark is sweeping through humanity. Every day, more people are waking up in the morning discovering they can fly, or bend steel with their bare hands. But some people keep waiting for the spark to hit. Comic book artist Kyle Rayner, for instance, or newspaper editor Clark Kent. Even high school health teacher Wally West knows something is missing.
Inevitably, those who should have powers are drawn to each other, trying to solve the mystery until they remember: they do have powers. They are the Justice League!
Once the League is assembled, they find Doctor Destiny—a villain with the power to make dreams reality—held captive by Know Man, forced to create a world full of super-powered people, but one without a Justice League. Know Man is defeated, and he warns them that they alone will now have to face the great evil he was preparing Earth to fight.
Flash 80-Page Giant #1 (August 1998), Mark Waid.
A wheelchair-bound Wally West visits the Flash Museum in the rain, but it’s closed for the night.
Travel agent Wally West, still in Blue Valley, helps other people visit places he’s never been himself, his father living it up in the tropics and Linda completely out of the picture.
Wally tries to keep his parents’ marriage together, hoping that if his mother doesn’t learn of his father’s indiscretions, they might not split up.
Wally rushes to the airport, hoping to convince Frankie Kane to stay with him, arriving too late.
A teen-aged Wally tries to run away from home, only to be caught by his father before he can get very far.
Wally sits, watching on TV as the Flash presents his new partner, Kid Flash (who looks suspiciously like Impulse).
All these possible lives escaped, prevented by a random million-to-one accident, as Wally is splashed with lightning-charged chemicals and becomes the Flash’s junior partner.
One-shot (1999), Christopher Priest.
Frustrated as being defeated by Parallax during Zero Hour, time-powered Extant devised a new scheme to alter history. Knowing the Linear Men would undo any changes he made, he instead arranged for them to make the changes for him, picking a man who was fated to die, then staging a battle with the JSA, waiting for the Linear Men to save the bystanders.
The man he saved went on to develop a weapon so powerful it had to be tested in space. The result shifted the moon’s orbit, in turn altering the Earth’s orbit just enough that Abin Sur’s spacecraft burned up, preventing him from passing on his Green Lantern ring. Other changes occurred. Barry Allen was standing in the wrong place to be hit by lightning, and all the Flashes from him onward never were.
Except for Impulse. Already a time anomaly, he alone remembered the real world. Waking up to find other people living in his house, Max Mercury missing, and finding that his friends didn’t recognize him, he had to search for Wally West—who didn’t know him either! As he left to take his strange visitor to visit Barry and Iris, he called out to his wife... Angela.
With two Flashes powerless and Max dying, only Impulse stood in the way of Extant’s ultimate victory. And, true to the story’s title, he succeeded in restoring history.
Flash #164–167 (September–December 2000), Geoff Johns.
Wally West, the Flash, found himself in a nightmare vision of Keystone City, cut off from the speed force and captured by brutal police convinced he was a criminal. Their records showed him as a teacher and track coach in Blue Valley. Broken out of jail by his world’s Captain Cold, the two of them tried to piece together what had happened to the world.
Without a speed force, Jay Garrick never became the Flash, instead dying in World War II. Without the Flash, the hero Mr. Terrific also died during the war, and America went mad with grief. Berlin was hit with the atomic bomb, and the JSA wiped out Germany, then retired in shame. The lack of a Flash impacted later generations too, as Aquaman died on the JLA’s first mission. The Titans never had a chance to form, Speedy and Aqualad dying and Wonder Girl sent into a coma their first time out. After the Amazons hunted down the man who nearly killed her, America became a darker place, with “heroes” killing villains without trial, and cops doing the same.
Eventually Flash and Captain Cold determined that they were in a mirror universe, and once they found Mirror Master they were able to piece enough together to escape.