The Vampire Diaries has adapted some creatures from the novels beyond the core supernatural beings, but the Guardians are among the few supernatural species that have been cut from the franchise so far - though Legacies works to set them up. Legacies went a step further than The Vampire Diaries and expanded the lore to include many other mythological creatures in the world, including gods, dragons, and a dimensional-consuming golem named Malivore. The Guardians are benevolent angelic beings who, like the gods, come from another dimension. The origin of the gods created precedents for the existence of beings from other dimensions.
The powers of the Guardians - which include elemental manipulation, time manipulation, telekinesis, and flight - are no longer out of the norm or the realm of possibility, particularly thanks to the Legacies series. Even Lizzie has boasted that time manipulation is a power of the gods, though it was never confirmed. While the witches were intended to remain neutral on Earth, it was the Guardians who served to protect against evil within their respective realms in the novels. However, their absence throughout The Vampire Diaries universe may have been strategic due to their overpowering nature and their otherworldly origin considering the humble, small town beginnings of The Vampire Diaries.
Why Guardians Couldn't Be Introduced In TVD Before Legacies
The dimensional expansion beyond the Other Side and the Ancestral Plane began with the introduction of Vampire Diaries Prison Worlds that trapped heretic witches. Other dimensions introduced include Arcadius' Hell, Bonnie's dimension, Peace, Malivore, and Limbo. The Guardians may have been too foreign of an entity to introduce in The Vampire Diaries show, even with the introduction of Hell in the final season.
It would have also been unnecessary to introduce the Guardians before Legacies because the dramatic conflict was intimately tied to the characters' relationships and abilities. For example, the blood of the Guardians, which can destroy any evil and prevent their resurrection, would have been useful to kill many villains. However, such an easily accessible and predictable "magic bullet," would not have been a sustainable entertaining plot device and would have stunted the characters' growth. The possibility of resurrecting characters who have died, or the search for a means to destroy them, will always be a good source of conflict to dramatize, evidenced by every time Alaric died in Vampire Diaries.
How Guardians Can Play A Role In TVD Future (If It Has One)
For the Guardians to play a role in The Vampire Diaries future, the Guardians would need an equal and opposite foe or weakness. None of the antagonists thus far could pose a threat to the Guardians if adapted faithfully as a species. However, in the novels, the Guardians' blood was not only a weapon against evil, but also irresistible to any blood-sucking supernatural. Within this lies the beginning of conflict that could be played up or adapted in a way that provides the Guardians both a foe and a weakness - which would work to counter some of the issues with them.
Legacies sets up another Vampire Diaries spinoff where the Guardians could struggle with an internal conflict similar to the one that plagued Hope: defining what it means to be a hero and doing good. For the Guardians, it could also be a matter of redemption or exercising justice versus retribution, which would make them being powerful less important or potentially troublesome for the series. This conflict could be enhanced if certain Guardians lived previously as a vampire, werewolf, or witch, further complicating their respective plots.
Earth Guardians who blindly defend against, if not also hunt, vampires and werewolves simply because their species were deemed categorically evil may stir conflict with the witches. This could lead some witches to aid certain vampires and werewolves they have judged favorably by their character and not by their race. While Legacies season 5 is cancelled, it is plausible that the increase of supernaturals could warrant a future intervention by the Guardians to ensure the protection and prosperity of humanity while further suppressing the increasing number of evil supernatural beings in The Vampire Diaries universe.