One thing I always found interesting was the suggestion in the 5e campaign Rise of Tiamat that if the players didn’t beat the Dragon Queen in the climactic final battle, then you could pick up and play in a world where the evil goddess stalked the lands, playing the resistance picking up where the great heroes of yesterday fell.
I doubt anyone has actually done this. But my group did a fun thing where, to win the support of the Metallic/Good Dragons, we had to go through a ‘Days of Future Past’ style dreamquest where we were doomed to lose the final battle - a test to see at what point the mortals would sacrifice Faerun to save the World by using the divine WMD given to us by Bahamut. Was cool to have disaster play out, even if it was just ‘all a dream’.