1. every new expansion should have 2 new classes MINIMUM. e.g. druid.... amazon.... assassin ....
2. add world boss raiding kind of like what was demonstrated in D4 demo. Also dungeon raiding. colloseum PVP, and also world PVP zones. So most of that farmed gear will be mostly for pvp, or if you just want to participate in higher end raids. Also to keep end content fresh, to have new areas added, new bosses, new side quests, new gear etc....
3. the stuff they talked about adding in D4.
4. the design for d3 was more frequent legend drops so that everyone gets a chance to use them. But i prefer that they LOWER the drop rate to make getting an item MORE meaningful. And also reduce the chances of just anyone having the best item straight away. You shorten the longevity of the game if you made everything drop left and right so ez. In D4 they are capping 1 mythic item able to be equipped, so that is interesting.
Ultimately this is pointless. D3 was not perfect, lets get that out of the way. But it did improve on graphics (regardless of our thoughts on the more colorful and brighter palletes use vs d1/d2), and flexibility in experimenting with builds without punishing the user by locking them into a build permanently.
The question you should be asking is how would you improve Diablo 4 after learning from the mistakes in D3, for what needs to be done to improve upon that.
The D4 darker horror theme is a good start. But i feel the microtransactions is a very bad thing for the diablo series.