AU Masterpost
Chasing Fire: An alternate universe in which, among other things, Tigerclaw and Ravenpaw first encounter Rusty and Tigerclaw drives him out. The ripple effects of this change are catastrophic for the forest territories, as the one cat who was destined to save them unknowingly leaves them to save themselves.
Tiger's Clan: An alternate universe in which the dog attack in A Dangerous Path goes far, far worse. With ThunderClan leaderless and without any one with enough authority to proclaim themselves leader, Tiger Clan dominates the forest territories until powers beyond anyone's control tear everything apart once again.
Jutpa's Clans: A complete rewrite of Warrior Cats that acts as the backbone to all other WC AUs.
Starfall: An original work of fiction with elements of Warrior Cats, but with a completely original story set in the aftermath of a realistic nuclear war. No mutants, no excessive radiation poisoning, just a whole lot of catastrophic climate change causing a mass extinction event and how humans and cats alike cope with this sudden event.
Rearranged Clans: A swap AU of Warrior Cats centering around the Clans swapping places. River Clan swaps with Thunder Clan, Wind Clan swaps with Shadow Clan, Blood Clan swaps with Sky Clan, and the Lake territories swap with the Forest territories.
Early Reunion: An alternate universe in which the cats Silverpelt sent out on the Midnight quest never return, with even their spirits having gone missing. With the situation rapidly deteriorating in the forest territories, Firestar leads them to the Gorge where Sky Clan resides, settling down in the surrounding areas.
Same Mistakes: The first arc of an unnamed post-canon AU of Warrior Cats set over one thousand years since the events of Into the Wild. Over countless generations, the history of the Clans has been lost to time, as the Clans evolved into Kingdoms ruling over a territory far from the Lake they once called home (if Changing Skies isn't a blatant ripoff of TNP then some other arc will be). There's a bunch of fun worldbuilding going on in this AU based on the cats spontaneously being freed from the canon and being allowed to do whatever the heck they want (e.g.: Sky Clan is fed up with the Forest Four's bs and returns to the gorge).