Assuming you trust a random Redditor, of course.
Diablo 4

A Redditor with a history of leaking some very accurate information on a number of titles, is at it again. Reddit user PracticalBrush12 just posted some of the first details on Diablo 4 just a day ahead of the start of BlizzCon 2019.

To put some things into perspective here, PracticalBrush12 provided very accurate details for Persona 5S, provided the name of Ghost Recon: Breakpoint before it was announced, and even provided info about Pokemon Sword and Shield ahead of its announcement. It is because of the diversity of their leaks that many believe PracticalBrush12 is being paid to "leak" information about Diablo 4 along with those other games. That, or they work at some sort of PR firm. Whatever the case is, you are free to believe or disbelieve anything they say. I'm merely relaying what has been said.

This user says that Diablo 4 is going more for the "dark, gritty, and gross" look. There will be "no more rainbow shiny bullshit this time," a nod to the more colorful appearance of Diablo 3.

Loactions are toned down, infrequently laid with corpses, wasteland, greyish deserts, clut caves surrounded by flesh, rotten crypts, marshes and bogs, plagued cities you name it. They are also seemingly much bigger, maybe even open? Characters now are able to ride horses (possibly other mounts), also there's contextual interaction with environment ? - climbing a wall Lost Ark style. Blood splatters look more like blood and not splashed jam too.
They continue on to confirm at least three classes. They are the Mage, Barbarian, and Druid. Mages use fire, ice, and lightning attacks. Barbarian can use swords, clubs, axes, and kicks. Druids use lightning, wind, and can transform into beasts like a bear and werewolf.

There will be four-player co-op included. They are not sure about competitive PvP though.

Apparently Lilith will be the new Diablo, which might make some Diablo 2 fans happy.

As a final couple of notes, PracticalBrush12 says that they do not know the release date. They also go on to suggest that you do not pre-order Diablo 4. They, like the rest of us, have no idea how bad the monetization aspect of the game will be. That and, you know, Blizzard is a terrible company and all.