But Monk was already a class in Diablo 1.Blizzard tends to copy ideas between D3 and WoW. D3 got the monk in 2012. WoW got the monk in 2012. So let's run with that in this post.
Diablo 4 was announced in 2019. BfA was current. They didn't add new classes. But Legion did. Legion added a double-jumping glaive-user that shot laser beams out of its demon eyes. So we might get some sort of WoW demon hunter in D4 (yes I know the name is taken in D3, they can call it something else).
Another choice might be Tyrande's night warrior. That emerged in BfA. Its not a class, but it could be. She's basically a Star Elf. Maybe D4's version would be a Star Warrior, some kind of tanky class covered in stars.