There is talk about that Diablo 4 is in the works. It might even be true! While that likely means that launch day for such a project is about five years out, it is not too soon to start having opinions on what exactly Blizzard should do to make D4 a success. Thankfully I have many opinions, and I am not too shy to share them.
I think the simplest approach is to take D2 and D3 and talk about which parts of each game worked. I played a metric fuckton of D2 and a fair bit of D3 so I think I know what I am talking about here, particularly since I am not married to either game. I thought D3 made some major missteps but had some things going for it, and obviously D2 was made a billion years ago and much of its shine is nostalgic in nature rather than objective.
The critical thing that D2 had going for it was exploration and story. That is, the lack of a cohesive storyline and the ability of the player to wander this way and that while getting terribly lost was a huge plus. There were things in D2 that you had to do, whether it be as simple as killing the boss of Act1 before going to Act2, or as complicated as building the flail in Act3 by clearing out a variety of huge dungeons. The key thing though is that tons of the game was optional. Don't like a boss? You can probably skip em. Quest not interesting or have terrible rewards? Don't bother! Want to kill Mephisto 60 times an hour? Be my guest!
That freedom to do whatever the hell you want is *amazing*. Random maps and quests you can ignore are the absolute basics. D3 did the opposite of this and it SUCKED. Every quest was mandatory, you had to do everything in order, you never made a damn choice at any point, and this led not to adventure but to tedium.
Let me also say that Blizzard needs to take the trope of a villain's face appearing on your screen to mock you when you complete objectives and burn it with fire. It takes any sense of mystery away, ruins the feeling of sneaking up on the enemy, and makes a mockery of immersion. It serves no mechanical purpose and feels like a punch to the gut.
The world needs to be *open*. Fuck the script. Just have a few gateway points that I have to get through somehow and then let me figure out how to get there. Have a tower in the middle of nowhere that had a basement five levels deep. At the bottom there can be a boss that drops a rune! A useful reward, but one I can skip if I so choose. Also add in caves that the locals need you to clear out, and have them grant you a reward when you do.
Make the game HUGE. Have a town where the locals tell you where to go, and have a road you can follow. Of course the road is infested with monsters, but that is the fun part! But if you wander off the road, oh my, there should be forests with twisted trees and swamps and caves and monsters and everything. HUGE. Full of random stuff! I want to run off into the wilderness murdering everything I see and discovering weird little nooks and crannies and bizarre dungeons. Like Skyrim, but Diablo style.
This sounds like D2 is just the thing. But I don't want difficulty to work like D2. Running through three difficulty levels was ass. Getting to endgame and then running just one zone over and over was also ass. D3 had a great innovation in giving you more things to do after you have beaten the story and gotten to endgame grinding mode. Scattering rewards throughout the world for players to hunt down is a great way to keep the world and the story relevant - sort of like world quests do for World of Warcraft right now. Blizzard would probably want to keep people from spamming single events over and over so I think they might want to do something like they did in D3 to get people to do multiple things. Perhaps ramping up rewards from events after each event is completed would do the trick, or giving rewards for doing many events in a single zone.
The numbers on abilities and gear need to be considered with care. First, abilities need to be tuned so that they are all useful as you level through the game. Second, gear needs to improve you, but not make you twice as tough and do twenty times as much damage as happens these days in D3. People testing the game should actually be levelling through the game with every ability to figure out which ones are worthless so they can be made relevant. Not every ability needs to be great, but there needs to be a big selection of top tier ones so that players have a range of reasonably optimal choices.
No damn trading. Trading sucks. The game should be about mowing down hordes of monsters, not fleecing noobs. You want to fleece people? Play poker.
There should be complexity in gearing. Lots of gear types. Lots of things like gems, runes, sockets, enchantments. Huge numbers of ways to augment your character through gearing. Path of Exile has some examples, but there are plenty of others.
Fast paced combat needs to be there. D2 combat was, for many builds, super repetitive and boring. D3 is often slicker and feels better. Having a single button that you use for every situation and every monster is not great, and D2 often had that problem.
Levelling needs to be closer to D2. The power increase from 90 to 99 wasn't much, but the time and prestige were immense. Let's get back to that, rather than paragon level grinding.
Let me summarize:
Minimal plot, most quests can be ignored.
Many random events with rewards, mostly optional.
Variety of useful tasks at endgame.
Fast paced combat.
Many ways to incrementally improve items.
Complex gearing at endgame.
There is your ticket to infinity dollars Blizzard. Do with it what you will.