Diablo 4 has a lot of excitement built up around it. The next game in the storied action-RPG franchise has a lot of history that extends beyond just the games, which already have a lot of plotlines left to play around with. Diablo 4 has a wealth of content outside of the games to pull from, including books and comics. The expanded media covers a lot of the series' backstory, like the creation of Sanctuary and the conflicts between angels and demons that predate humanity itself.
A prime example of Diablo 4 already pulling from the well of lore available is its choice of main antagonist. Bringing Lilith back into the world of Sanctuary has huge ramifications and leaves the possibility open for Diablo 4 to explore some of the events that aren't referenced heavily in the previous games. Lilith is one of the two beings that created Sanctuary, the world where the games take place, and her children went on to be vitally important to the history of Diablo. Her presence could be what opens the floodgates to other lore, and she only represents a small part of what could be coming.
Lilith is hugely important to the overall lore of Diablo, but she only represents one side of the Eternal Conflict. While the events of the previous games have changed the world in serious ways, Diablo being defeated (for now) among them, it seems like Diablo 4 may lean heavy into its past. The new class-specific quests in Diablo 4 might be a way that this is done. Having the different classes interact with members of long-running factions like the Priests of Rathma or the Crusaders could be a good way to both move the narrative forward while also referencing game's past. Especially with Deckard Cain dead, players will need new places to learn more about Sanctuary's deep history.
A large chunk of the expanded media focuses around the Eternal Conflict and the Sin War. The former is more well known to casual fans, and the battle between Heaven and Hell has long influenced the overall story of the Diablo games. The Sin War is really only dealt with in the books and comics, but is equally important. Lilith and the angel Inarius had children together: the Nephalem. Nephalem soon grew to become more powerful than the angels and demons that created them, and that led to a war between factions that saw them as threats to Sanctuary. That conflict spread and could have big implications in Diablo 4.
It's possible that long-running tensions that have been explored throughout the various expanded media could come to a boiling point in Diablo 4. Angels are powerful forces for good, but often only their own version of what good means. Many of the angels believed the Nephalem should have been destroyed during the Sin War, and their return to prevalence in Diablo 3 could lead to a conflict with Heaven itself.
That might seem far-fetched, and it's just one of the many possibilities for Diablo 4. There are so many threads left to pull, even after most of the Evils have been defeated. Hopefully, fans will get a more comprehensive look at Diablo 4's story soon, as it definitely has a deep well to draw from.
Diablo 4 is in development for PC, PS4, and Xbox One.