Well funny enough julie said she had planned for tvd even after the salvatores left and im not going to hold that to her, she may have changed her mind, fine, she is human like the rest of us

But at the same time even im interested in those backup plans she told about

Honestly it would have been difficult to find out what they could do, they stretched out every possible scenario with the mfg except i think delenas child stefanie salvatore could have led to something interesting cause she would presumably have the blood of the doppleganger and the cure, she wouldnt be a powerful being but would be a valuable supernatural tool, she'd basically be like a combination of elena and katherine when they were both human but it could have led to interesting faction war that didnt take place in new orleans but one section wanted the cure for themselves, the other wanted elena alive to use her doppleganger blood to end the vampire race or create a new original

Other than that, i have no idea what else they could have done with the few characters they had left

I dont think they will ever get into the origin of the witches that the fans want to see

They could have shown the beginning of the school. How they created the rules, how they went to search for students or teachers and to convince them, as we know since the episode 1x14 of Legacies they exist others school of magic, they could have shown if Alaric contact them to for help him.

They could also have shown how Damon is adapting to his new human life without his brother.

Show Bonnie's travels, if she's gone see how magic is elsewhere or if it was normal trips.

But a few episodes would have been enough for that, a season 9 or 10 would have been too much. How already said, they had exhausted all possible subject.