So I have a LOT of AUs, not all of them get talked about! A little list if some of the ones I like.
Cloud!Three - Leafpool and Crowfeather leave permanently, having their children in Skyclan. Cloudtail and Brightheart have their second litter earlier, Ambermoon, Snowbush and the renamed Dewrush as the Kin of Firestar's Kin.
Prophecy!Scourge/Tiny!Fire - Bluefur takes Tiny into Thunderclan, only to realize he is the subject of a Prophecy: The Night Alone will save our Clan.
Anthro AU - Exactly what it sounds like but every character exists all at once, mostly just for fun.
Power of Stars - Rather than being Firestar's Kin, there are multiple sets of Prophecy Cats, all related to the OG founders. Lionblaze, Hollyleaf and Jayfeather for Windstar. Heathertail, Galeshine (Galekit), and Harespring for Shadowstar (through Sun Shadow). Minnowtail, Willowshine, and Cloudsnap for Skystar (through Blackclaw, Primroseheart and Reedwhisker's dad). Flametail, Tigerheart and Dawnpelt for Thunderstar. Things get complicated when Skyclan shows up early for OOTS, bringing along Firefern, Harrybrook and Stormheart for Riverstar.
Warriorclan joins the Clans - Very self-explanatory. They come with Graystripe in the end, settling in a more remote spot in-between Sanctuary Lake and The City.
50% left version 1 - Great Battle was far more disastrous, every cat barring few has a 50/50 chance of getting killed.
50% left version 2 - Post-Avos. Darktail's massive fire along with a horrific storm puts all cats barring few on the chopping block.
Lakeclan - As The Great Journey comes to an end, the majority of Clan cats cannot bare to be rigid, separated groups once more. The Lake is governed by the 4 Leaders at respective camps, but borders are a thing of the past, cats are completely free to go where they please and get close to who they want. Of course, not everyone agrees with the new arrangement...
My Clangen AU - If I could draw, I would have made a comic series for these guys. A newly formed Clan, Moonclan, fights for its place to survive amongst the already pre-established Songclan, Leafclan, Moleclan, and Weevilclan. (Each Clan is tracked through alternate Clangen files, each with a different biome. Leafclan is Classic Forest, Moleclan is Tunnels, Weevilclan is Shipwreck, Songclan is Wasteland, and Moonclan is Crystal River)
Feel free to send asks about any of these AUs!