One of the absolute biggest reasons why a lot of people consider D3 a failure is because they decided to take everything from Diablo 2, and do the reverse. And the classes weren't spared. I mean hell, they originally wanted D3's Barbarian to be a, uh, idk I forget the name but it was along the lines of a Savage or something, before they realized it was so obviously the barb they had to change his name back. And how many of you would have liked to just see the Nerco in the first place, instead of the Witch Doctor?
Diablo 4 would need to, in my opinion, go back to their main classes at least; Nerco, Amazon, Barbarian, Paladin, Sorceress. However, like I've said before, Blizzard has a lot of work ahead of them. They can't release an empty game like they did with D3, and plan to add in actual end-game later. There would need to be more classes as well, and I'd personally pick at least the Druid, and I've love to see the Sin again. But I also think the Monk is unique enough to still fit alongside the rest. Essentially my vote is for the D2 LoD roster as a bare minimum.