The trial of Ben Solo
So in this scenario, Ben Solo still returns to the Resistance base with Rey on Ajan Kloss. He is met with very little enthusiasm and is arrested by Poe and Finn. He is placed in some sort of prison, which he doesn’t resist.
This ending would be a little bit like Return of the King where we would jump into the future. We would see a makeshift trial where Ben is accused of mass genocide and murder. Perhaps he’s defended by Rey, and she can plead that he was seduced by Palpatine and it wasn’t actually him doing this. She can also state how he already redeemed himself by taking part in Palpatine’s destruction.
Either he would get off scot-free or he would get sentenced to life in prison. If he were sentenced to life in prison, that would be just as a depressing ending as him dying, though it would give him the chance to be freed at some point in the future, perhaps helping the Resistance/New Republic against some future threat. The problem with this is that it would leave the Skywalker saga open to more stories, when Episode IX was supposed to be the definitive and final chapter.