The Reddit leak confirms that Diablo 4 is essentially a return to roots, with a dark look inspired from Diablo 2 and combat mechanics borrowed from Diable 3. There's a strong possibility the environments are open-world, as the characters appear to be able to ride mounts. Possible classes include mage, barbarian and druid and Diablo is replaced by Lilith.
https://www.notebookcheck.net/New-Diablo-4-info-leaked-ahead-of-BlizzCon.440684.0.html