Many years ago I have send an email to Blizzard telling them the idea for the story of Diablo III. I have seen great parts of the story being introduced in Diablo III. From an account email@example.com which I dont have access to, anymore.
What I have analysed regarding human behaviour and psychology tells me that Diablo 4 needs 3 important things to become Legendary.
Drop rates and all the good items and runes to be so hard to find that you have to spend months to find one or two. This keeps the people entertained, they will never give up. They will build an addiction to the game and will worth dedicating all their time. If the monsters are easy to kill, items are found fast, then what is the point of waiting so many years for just a few months of playing. Please make it very difficult. No need even for new patches with new things introduced if you make the drop rate very low and hard to level up and super hard to defeat bosses.
Human beings never like it when they get something easy. They do not value it. They always want something else after having it. Ask psychologists.
Would be nice to have more feelings while playing it. Excitement of finding something amazing appears with my previous point. But would be more fun to be a little bit scarry sometimes. The design of the monsters/songs/cinematics. If you can design the game in such a way. Would become a huge event on the planet.
My personal preference. I would like to not have the season restarted so fast. Let me struggle a lot to find things and then let me enjoy the glory more with new commers. If everyone has every item and rune and built easy then the game will be just at its 10% success.
Please give me yoir feedback regarding what I have posted. I wander what you guys think about it.