This is kind of cliche but time travel (fix it or ruin it further) AUs are my favorite. There are quite a few out there. Otherwise I also like those where Firestar joins a different Clan or does not end up being important in the story at all with someone else taking his place to figure out what to do in stopping Tigerclaw/star.
I recently found and started reading an AU where Ravenpaw is the protagonist of TPB and Rusty never joined ThunderClan in the first place. It's really good but since the last update was in 2019 for
Forest of Secrets (renamed to
Hidden Shadows) I'm not sure if it will even be continued at this point. Here it is:
archiveofourown.org/series/715845When it comes to AUs I have created for myself or based on the general idea of others, there are a bunch as I'm currently in the process of collecting those I wrote down somewhere into multiple notebooks. One is already filled to the brim with different ideas. Here are a few:
Fireheart's littermates find and join ThunderClan later in TNP or sooner with him (there are two versions)
More FireSand litters than just one (two sons get born later in PoT)
Spottedleaf survives Clawface and steps back as a medicine cat to later become the mate of Fireheart (different kits)
Fireheart is the one in a forbidden romance with Silverstream (there are once again different kits than in canon)
Gorsepaw lives after Tigerstar attacked him and later becomes leader of WindClan instead of Onewhisker
Snowkit escapes the hawk trying to grab him and later chooses to become a part time warrior and healer
Brambleflower (Brambleclaw as a healer)
The Brothers
Breezepelt's littermates survive
Element Clans
Warrior Cats as Humans