Diablo IV is everything but confirmed for BlizzCon 2019. The artbook leaked by Amazon Germany just confirmed that the game is coming, but new rumours give a lot of details about content, design and art style.
A Reddit user with a good track record of past rumours says that "Diablo 4 is actually going to look dark, gritty and gross". He depicts toned down locations and different biomes like caves, crypts, wastelands, deserts, marshes and plagued cities. The general inspiration comes from the "nitty-gritty dark style" from Diablo 2.
A franchise first would be mounts. At least, the user states that we will be able to ride horses which would be very suitable for the new, bigger maps, and he is not sure whether the main scenario is open-world or not. There will be also "contextual interaction with the environment", but the user doesn't know how it will work. And, finally, he talks about 3 classes: "mage (uses fire, ice and lightning so far), barbarian (swords, clubs, axes, kicks etc), and druid (lightning, wind, and transforming into beasts - so far bear and werewolf!)."
Later on, video games analyst and ResetEra cofounder Daniel Ahmad said that these bits of info are similar to what he has heard about Diablo 4. And added that the mobile game Diablo Immortal will be at the show.