Although Diablo and Darksiders are completely different games in terms of genre, the two franchises still touch of the great mythos of heaven and hell. They do so in different ways, but it's interesting how much similarity there is. In fact, it would be interesting if Diablo 4 took inspiration from the Darksiders games and used some of its shared source material in a similar approach.
Of course, this shouldn't be in a "copied homework but different way," yet with Diablo 4 tackling the mother of Nephalem Lilith, it's prime time that the game expanded on its lore of Nephalem. It's worth mentioning that Nephalem are seen as the ancestors of mankind in this lore, but in Diablo 3, each character was still a Nephalem.
Kalmor can be met as well, serving as the last living Nephalem of the pre-Sin War era, and it's clear the race still matters. With Lilith at the fore of the next installment, it would be interesting if Diablo 4 took a page from Darksiders and brought in the Four Horsemen of the Apocalypse...somehow.
Diablo 4 Horsemen
Obviously, the Four Horsemen of the Apocalypse Conquest, War, Famine, and Death manifest slightly differently in Darksiders, and it would be hard-pressed to have these Four Horsemen appear in the game. However, a "Horseman" class would likely work well and take a lot of different directions based on this character. Famine and Death could operate as some status effect or theme, while Conquest and War could manifest in the melee capabilities of the class. To cap it all off, some sort of summonable horse could see Diablo 4 play a lot like Darksiders Genesis.
Notably, given the shared source material of The Old Testament's Nephilim, this character wouldn't have to be a Nephalem like Darksider's War, Strife, Death, and Fury. It could just be that this class is called forth or wants to take down Lilith—comparisons would be inevitable—but it would be interesting to see Diablo's take on the Four Horsemen of the Apocalypse, however it manifested. Given the End of Days, the Destruction of Heaven, and other plot points of the Diablo franchise, it's a bit surprising that it hasn't really been done.
Not to mention, if the Horsemen was a straight-up evil class in Diablo 4 (pursuing Lilith for whatever motive), it would be a first for the franchise. There's been anti-heroes, there's been dark-themed classes, but there have never been known that really gave into the dark and demonic offerings of the franchise. With Lilith as the presumably main antagonist of the game, there's never been a better time. The Horsemen could pursue her for entirely demonic reasons, as she made an enemy out of everyone, and the game could explore the nephalem further, with or without the Horsemen as one of them. Either way, Diablo and Darksiders share similar themes. One notable difference is Darksiders lighter take on the happenings, so a Diablo-style Horsemen would be interesting, nonetheless.
Diablo 4 is in development and confirmed for PC, PS4, and Xbox One.