Mixture of easy and time consuming to keep you playing:
Most would be multi-tiered (So Kill 500, 1000, 5000, etc)
> Kill x Enemies (Multi-tier)
> Open x Chests (Multi-tier)
> Obtain all weapons
> Obtain x Currency (Multi-tier)
> Die x Times (Multi-tier)
> Beat the game without dying (Softcore) and Hardcore version would be from beating the game [obviously as you can't die
]
> Kill every enemy and the boss in the game (Top Of The Food Chain)
> Die to every enemy and boss in the game
> Descend to a lower level x Times
> Beat the game in under x hours (Speedruns!)
> Reach the maximum carry capacity x times (Pack Mule)
> Beat the game on all difficulties (if this becomes a thing)
> Beat new game + (if this becomes a thing)
Just a few ideas for now. Most are basic grinding ones, but I find they can keep people playing/get people interested.