The Originals was a successful spin-off of The Vampire Diaries, with more adult-like characters and themes. In The Originals Klaus Mikaelson grapples with becoming a father and a better brother. His past haunts him, quite often, but he works to embrace his heroic side, despite his tendency to be a villain.
A tale of family loyalty and redemption, it echoes a similar spin-off, Angel. Both are set in a more adult setting with older characters and a main character that seeks redemption. The Originals gave us five solid seasons where we were introduced to a variety of characters and storylines, not all of them resolved.
Tristan and Aurora
The Mikaelsons, new to immortality, meet Tristan and Aurora, a brother and sister who represent a more cultured, sophisticated lifestyle. Both become vampires, turned by Elijah and Rebekah. Aurora, imbalanced but beautiful, is Klaus's former flame. Tristan serves as a leader of the Strix, a 'civilized' group created by Elijah.
Both set out to destroy the Mikaelson family in Season 3, but in the end, the Mikaelson family prevails. Klaus walls up Aurora in his house. Tristan is sent in a container in the sea--to live and die, multiple times. As we have seen in The Originals, characters that are in similar predicaments can be brought back. Both Tristan and Aurora weren't, although they remain threats while alive.
The Harvest Witches
In Season 1, the witches complete a ritual called The Harvest where four of the strongest, most promising young witches are killed to restore power and magic to the coven. Later, we find that the four young witches will be resurrected, becoming more powerful upon return due to the time spent with their ancestor witches. Davina, refusing to be killed and under the protection of Marcel, stops the other girls from returning to life, and contains all the magic. Each time one girl died, her power transferred to the remaining witches, leaving Davina with the power from the other three witches.
When Davina dies to complete the ritual, eventually all four come back. While the series does make use of Monique (made to be the villain) and Davina (Monique's better half), both Cassie and Abigail receive little screen time. Cassie serves as a vessel for the powerful witch, Esther (the Mikaelson matriarch) to return, but we don't get to witness much of Cassie's power and even less of Abigail's. All the Harvest Witches were chosen based on their potentials. It would have been nice to see more of their capabilities. Davina has been brought back many times in the series. Why not bring back the rest of the Harvest Witches again, especially the two that came across as duds, rather than dynamic.
Mikaelson Witchcraft Ability: Kol and Rebekah
The show suggests that if Kol and Rebekah had not been turned into vampires, they may have become strong witches. Both possess the potential to do magic, and Kol feels like it was part of his identity denied to him by his mother and the vampire curse.
Kol's infinity for witchcraft could be what attracted him to Davina; he could be 'living through her.' That dynamic and Davina's response to it would have been an interesting one to play with.
Antoinette
Poor Elijah and his many doomed loves, of which Antoinette was one. While Antoinette escapes with her life, her mother (someone recognized as a fanatic) is killed. The fanatical "pure" vampire moment turns to her to be their leader, a position that she doesn't embrace. In Elijah's time with Antoinette, he was presented with another way to live: Embrace being a vampire.
Antoinette chooses to live as a night creature, rejecting daylight rings, stating that she is no longer human and can't pretend that she is other than a vampire. Still, being a vampire doesn't mean overt violence. She shows Elijah how to make the victims be calm and happy while she drains them, making the blood taste sweeter. This is a lesson that all of the Mikaelsons could use. Antoinette leaves after her mother is killed and her brother spared. Where does she go? How does she repair her broken heart? Does Elijah mourn the loss of her and his simpler lifestyle?
Klaus's Werewolf Roots: Biological Father and Family
Klaus does meet his bio dad, who is a nice person, but Klaus-being-Klaus eventually kills him. In doing so, he accepts that Mikael is his father. However, we don't see more of Klaus's werewolf roots. There must have been other family members and descendants. Wouldn't they have sought him out or wouldn't he have sought them out?
After pursuing his werewolf side for so long in both shows (The Originals and The Vampire Diaries), we doubt he'd be satisfied until he met more of his pack. The hybrid army was given up, too. Do these examples show that Klaus changes his focus easily?
Elijah's Sires
In the last episode, Elijah sacrifices himself so that Klaus doesn't die alone. It is an incredibly sad moment for two reasons: Death of beloved characters, and Elijah's refusal to live a life outside of his brother's. However, won't Elijah's sires die? Klaus's sires are no longer connected to him, but Elijah's sires are still connected. Hopefully, Elijah's specific magical death prevented that from happening.
Freya and Her Aging
When the evil aunt, Dahlia, was alive, Freya and she were bonded, causing their magic to be stronger. They also would fall into a sleep to maintain their age. Without Dahila, it seems that she is not as strong of a witch. She had been immortal, but over time could she age?
Freya, though talented, is now more limited. Freya's identifies as being a powerful, immortal witch. Any change in this identity would have created a crisis for her. Freya continues to try to save her siblings from various dangers while they don't acknowledge what she goes through.
Camille's Brother and Family
Camille's backstory always seemed to be unrealized potential. Her brother, possessed, killed others. Her uncle also became possessed and dangerous.
The family stored black magic objects. They were the human representatives on the keeping the peace/supernatural & human council. Yet, despite this, so little is done with their storyline. Her brother and his story, especially, is unrealized potential.
Sophie
Sophie's storyline abruptly ended. In Season 1, Monique (Sophie's niece) kills her, showing that Monique is not the innocent niece that Sophie remembers.
However, Sophie is literally connected to most of the important players in Season 1, and while other former witches have come back many times, she doesn't. Her family loyalty and that betrayal felt cut short. Her storyline could have served as a mirror for Elijah, being betrayed by his family.
Insane Witch Asylum
Rebekah (in Eva's body) is sent to the asylum, where she meets Freya. Kol's witch friends, Mary and Astrid, were also sent there. Rebekah and Freya leave. We don't know what happens to the remaining witches.
Did Freya free them all when she opened the gate? Are they still there? After being isolated for years, how is their mental stability? This was an interesting part of the series that could have involved more big bads, but it didn't, and The Originals moved on too quickly. Maybe these unresolved storylines will come to play in Legacies.