These days achievement-based rewards are considered awesome. You want more than just the grind and grind and grind to get itemX.
Specifically I'm contemplating along the lines of:
Finishing a level:
a) Without taking any damage.
b) Using only a single weapon.
c) Not picking up a single drop.
d) Using only vials.
e) Not killing any monsters. (clearly wouldn't work with all levels)
And there being a level-based reward.
Complete a Wolver den only using a single weapon? Not a big deal.
Complete a Haunted Passage with a single weapon? More difficult.
Complete an Arena with a single weapon? Quite a lot more difficult.
* Defeat Lord Vanaduke with a single weapon? Yay, a 4-star item for you.
* Defeat Lord Vanaduke with incomplete equipment? Congratulations on a 5-star material.
* Defeat Lord Vanaduke without taking any damage? Congratulations on a specific, pretty awesome 5-star piece of equipment.
Etc.
Knowing that such things require tracking, contemplation of implementation is best done early. This would also give people something to chat about.
That just sounds painful.
And by painful I mean, if I had a samurai sword I would be using it to turn my guts into sushi and still be having MORE FUN than I would playing SK.