[Crazy Jetty]

The Exiles is Quantum Leap meets Sliders, meets Marvel Superheroes.

It began with Age of Apocalypse Blank becoming displaced in time, where she met alternate reality versions of Morph, X-Man, Nocturne (The daughter of Nightcrawler and Scarlet Witch in her home reality), Thunderbird, and Mimic.
All displaced from time, they are told by a being only they can see and hear known as the Timebroker, who tells them that something has gone wrong in their realities, and they cannot travel back until they fix it, by righting wrongs in other realities.

It's indirectly connected to X-Men, but not exclusive to Marvel's Merry Mutants. Most of the time they help many other marvel characters. A female Sunfire, a female Sasquatch, Magik, AoA Sabertooth, Hyperion, 616 Psylocke, and 2099 Spiderman have all been members of the team at one point or another.
And composing a second team once, was an evil Spiderman (Carnage, I think?), She-Hulk, Gambit, Storm, vision, and Deadpool, too. Weapon X came into conflict with the Exiles many times, because their mission was to be a hit squad of sorts, and kill bad elements in order to fix timelines.

It was one of Marvel's better titles, constantly getting good reviews. And far better than the actual X-Titles. Naturally Marvel couldn't leave it alone and royally screwed it over, pretty much destroying it at the end.

It took the basic concept of "What if..." and added a stable cast of characters (Great characters at that), and let it grow.