1)BOSS FIGHTS The player should find out the the secret involving how to beat the bosses. Secret could be environmental puzzle in boss area or lore based. You need to learn from somebody or find out in the journal what is the weakness. Every boss involves different strategies to beat. ->RISK AND REWARD : thougher the boss is, the better the reward is.
2)HIDDEN BOSSES AND HIDDEN AREAS Some areas are locked until you find special item or solve some quest. Items could be rogues stones or some other portal keys we haven't seen yet. Bosses could be liches, demi-liches or some other new entities with backstory.
3)NEW GAME + MODE Player could have his all items but with increased difficulty. In this playthrough items could be upgraded to the highest level but challenge should still be there.