AU where Thunderclan is successfully driven out during eclipse
Hollypaw is injured and heals (in secret) in Shadowclan
Thunderclan joins Skyclan at the gorge
Hollypaw is too injured to speak, so Shadowclan doesn't find out until the next gathering.
Wind and River suggest shifting the borders now that Thunderclan is gone, Shadowclan's lack of involvement and knowledge of the attack allows them to claim some territory as well, and grants Riverclan to the rest of the marshes.
Hollypaw wakes up in Thunderclan camp and thinks that her clan won the battle. Then she sees Littlecloud in the medicine den, but it doesn't fully sink in until she rushes into the clearing and can only smell Shadowclan.
At first Holly is terrified that she's being held prisoner and that Shadowclan fought against Thunderclan. As it turned out, they had been waiting for Hollypaw to heal so that she could join the search parties.
Blackstar insisted on taking most of the woods around the old Thunderclan camp and occupying it so that Windclan wouldn't get any ideas, while also maintaining use of the Shadowclan camp.
Blackstar's plan is to locate Thunderclan quietly and bring them back, then make a show of supporting their return at the following gathering. (Outwardly, his attitude is very much "Oh sweet you're back, yeah we kept your nests warm for you. It's great to still have four clans!)
Skyclan returns as well, which sparks the border arguments early, but it's also 3 clans against 2 which works out great in the short run.
Hollyleaf warrior of Shadowclan??? Yes please