If a new expansion is to cost as much as the game, then it should be as big as a stand-alone game.
We have Sanctuary and its protectors. Now we need Heaven and Hell.
For a future expansion, an entire world must be made of Hell. You can select one of the classic types of enemy as your character:
Skeleton Mage
Corrupt Rogue
Fallen Shaman
Oblivion Knight
Goatman
We play from the perspective of Evil.
Factional PvP etc… Sigh. Could be so fun.
I feel like it would be easier for you to make your own game at this point with all of the suggestions, reworks, alterations, etc. you’ve been making with your posts. Get yourself a small indie team, of roughly say 90 people, and just have fun.
I want to play as a Necro Minion. Then when the Necro tells me to fight.
I will say “No I am on break” 
Or he could just work for the D4 team
Its pretty easy. Just think up something fun and then create the opposite.
Being a Necro was so much better in the old days (Diablo 2). You could demand the skeletons work 12 hours straight with no breaks and sacrifice themselves to the fires of DIablo.
Now though, with all the skeleton labor rules I am lucky to get them to kill 3 goats before break-time
./sigh
Yea the only problem was getting them to attack the same enemy lol.
Larry stop chasing that Buzzard and focus on this fetish attacking me! No don’t drink the water, you’re a skeleton you idiot!
Having been an indie developer, you don’t need 90 people. lol.
It’s not as hard as you think. The hard part is the networking and foundation. Both of these have been completed to an extent. All of the physics and “beauty” assets are complete, e.g., volumetric lighting, post-effects, etc.
It’s the reason why Call of Duty is able to launch a new game every year. It’s the same code, but a simple reskin / remap.
Mapping / story / quest design is INSANELY easy.
So what you’re saying is we can expect a game to come out of you soon? Eh? Eh? I’ll demo test it for you.
If I had the code / access to their engine, yeah. I could do it alone in 1 year. No cinematics, though.
You will be shocked at how easy it is to make a game. Heck, people today can make games without any coding at all these days. You will be limited to what is included in the asset, but it’s honestly quite incredible.
Here are just a few examples of what “no coders” can work with:
World Design: https://www.youtube.com/watch?v=wkypCxc2Mzg - This particular asset is dated, but there are so many like it that is up to date.
For combat, loot, storage, questing, etc etc etc: https://www.youtube.com/watch?v=ETN5iRb6T9E&t=76s
No no, I want your game, not the game you’re trying to alter. I want to see your version of a Diablo game made from scratch. 
You have so many ideas brewing up there for what would make the game better, but we all know Blizzard won’t pay attention to any of them. But if you made your own game, oh the freedom you would have, and everyone could play it too.
I honestly do plan on working on a RPG.
Currently been working on an “Undead Civil War FPS game”. The only footage I have is years old, so it looks pretty bad. Was working on weapon animations at this point against mindless bots. I’ll upload a current video soon.
It won’t.
They already made that game. That is how Diablo immortal is set up.