1) Adjust the total amount of enemies in each act:
1a. Current amount in Normal mode.
1b. Current amount multiplied by 1.5 in Nightmare mode.
1c. Current amount multiplied by 2 in Hell mode.
1d. Current amount multiplied by 3 in Inferno mode.
Increasing the amount of enemies to kill would greatly increase the amount of fun people have in each difficulty and actually making the difficulty modes different from each other.
2) Add a real secret level:
2a. Create a zombie cows level.
2b. Create an endless dungeon (guaranteed one legendary-dropping chest every 10 levels).
2c. Create a massive dungeon (20 levels, guaranteed legendary in level 20).
The thing that disappointed me the most about Diablo 3 is the lack of a substantial secret level. The Secret Cow Level was so amazing in Diablo 2 and there is no excuse to skip adding another one.
3) Improve Whimsyshire:
3a. Triple/quadruple total amount of enemies.
3b. Add unique/champion monsters and 4 bosses (only one boss randomly spawns each game).
3c. Triple the level size.
The secret level sucks, period.
4) Add Multiplayer dungeons:
4a. Create dungeons that are too hard to solo.
4b. Create dungeons that only groups can enter.
4c. Create multiplayer-only events.
Why do multiplayer right now? Multiplayer-only content is a major need for a game that is intended to favor multiplayer gaming.
From Shadout on the Diablofans forum
Maybe they could make endless dungeons, where there was like a 0,1% chance that it spawned a special "cow level" design in one of the levels
5) Create difficulty-specific, random spawning monsters that have a 20% chance to drop a legendary (very rare, only 3 spawned in random locations in each game):
5a. Normal mode - none.
5b. Nightmare mode - monsters named "Nightmares" that look like wraiths.
5c. Hell mode - "Hell Soldiers" that are fiery skeletons wielding swords.
5d. Inferno mode - "Inferno Beasts" that resemble minotaurs with large axes
My suggestion for random spawning monsters specific to difficulty actually betters the RNG system instead of breaking it. Let's say it takes someone an hour to find a legendary on average. Randomly throwing in three or four of the monsters (never two in the same area) in every act would give players an extra thing to look forward to encountering while grinding. Each of these monsters would have a 20% chance to drop a legendary, which, if you think about how long it would take to find one of these guys (maybe an hour), would give players only a slight increase in legendaries found if they are lucky enough to have the monster drop a legendary.
In higher difficulties, I see how this may be controversial, but in Normal through Hell modes, lower end legendaries would have a better chance to drop and it would give a little more excitement to the tediousness of leveling a character.
I might even suggest messing with the numbers a bit like so:
In Normal mode, 20 of these monsters spawn in each act and have a 20% chance to drop a (obviously low-end) legendary.
In Nightmare mode, 15 of these monsters spawn in each act and have a 20% chance to drop a legendary.
In Hell and Inferno modes, 10 of these monsters spawn in each act and have a 10% chance to drop a legendary.
Seriously, this whole repetitive story thing sucks, especially when every difficulty is the exact same, but slightly harder. This would at least add some uniqueness and something new to the difficulties for people leveling characters for the 3-100th time.
6) "Chaos mode:"
6a. In the menu for turning on/off MP levels, why not add another option for something like "Chaos mode." This mode would be exactly like regular questing mode, except all questing dialog is turned off. The world spawns completely random monsters instead of area-specific monsters and random act bosses have a chance to spawn in every area (especially areas where bosses do not regularly spawn).
This would bring true randomness to the game, and it likely wouldn't replace regular mode because pure randomness isn't always wanted when farming. Many people like predictable enemies for efficiency. This mode would give players who are tired of the same crap something to go wild in.
Source on Diablofans: http://www.diablofans.com/topic/82351-five-suggestions-for-substantially-improving-d3-content-wise/