How would you make the game harder?

Difficulty is not punishment. Difficulty is balance. As it stands right now, the players in D2 got massive buffs (Anni, Torch, Runewords, various other changes) but the Bosses in D2 never got balanced to compensate. The bosses have a variety of bugs and other issues that cause them to be seriously weak and underperform. This idea isn’t a punishment, it’s simply a balance adjustment to compensate for the players immense power creep over the decades.