Diablo 3 is too repetitive imo. So Blizzard should add:
1. Day-night cycle for all places.
2. Large random generated maps which players can more freely roam in.
3. Much more mobs (many new models).
4. An Endless dungeon that goes on till the player exits.
5. Ingame Graphics option to make the graphics Darker, and more Gritty. Or it automatically gets darker at higher difficulty levels. More dynamic lighting and shadows throughout the world.
6. Update the in-game textures so it's crisp. Now it's quite blurry.
7. More skill customization and randomization with builds.
8. A new Act, and the random map generator will make all previous acts much bigger.
9. Random Quest generator. It's boring to play the same main-quests a billion times. 10. More Co-op and PVP modes.
11. Maybe allow users to have their own home, that they can build, decorate with ingame items and defend from mobs.
If they add all that Diablo 3 will be much better.