I don’t know how the difficulty will be implemented in Diablo 4, but I don’t want to see any more difficulties from T1 to T1000 like on Diablo 3. This will unbalance the game absurdly on exp, gold and loot (legendary/set piñata) winnings.
However, I found the inferno difficulty of D3 vanilla brought a real interesting challenge. But as we know, the inferno was unbalanced because of AH, items 61-63 only found in inferno (which would complicate the progression), and that each Act became just a gear check.
I would like an equivalent of this difficulty to be implemented in D4. I’m not asking for the return of the Enrage timer or the invulnerable Minions. But I think it should be balanced so that it’s not extremely hard, and it doesn’t give too much benefit to the players who venture into it.