A few suggestions:

- Coop up to 3 players on the screen at once.
- Mobs get +125-150% HP per extra player. Better stat effect resists.
- Mob melee attacks hit all players on the screen. Attacks that target pets hit all pets on the screen.
- Maybe add some new drops that only drop in coop mode, etc some legendary tier equipment.
- Who gets the loot drops is completely random, no rolling for or arguing over loot.
- Have some of the healing skills and buffs multi-target in coop.
- Some uber bosses.

Of course, I have no idea how hard it would be to implement some of this stuff.

Fun to consider the possibility. It's possible. Even a 2-player mode would massively improve the enjoyment.