Add puzzles - Sure you character is limited to running around killing things and breaking stuff. But couldnt there be puzzles in the dungeons or out in the fields of sorrow where you have to break stuff in the right order? OR else a hoard of zombies rise every time you get it wrong? .. Or perheaps to open secret doors to hidden rooms where you find other content like :
Actual in game rewards for your achievements - I.E ... Legendary Weapon added to your inventory for beating the game on normal, nightmare, hell, inferno ect.... People would be more than happy to work toward a major long term goal that actually payed off hansomly.
Unlockable Character Skins - Change the appearance, posture, build, of your characters by unlocking rewards that allow you to do so.
More Explorable Territory - You beat the game the first 2 times and you quickly get bored of looking through the same basements your next time around, nothing changes.
A dedicated lobby where you take your characters to dual, or just inspect other peoples characters. Someplace in game. Somewhere where their toons can run around aimlessly until 4 of you get bored enough to go play the actual game.
These are just a few. I did not spend hours thinking of these ideas, just as long as it took me to type them, and some of them are actually plausable. So please blizzard, lets not overthink this. The games foundation is amazing, lets just not build a shanty on top of it and call it done. I want to see a work of art.