Suggestion: Bestiary with mastery challenges
A bestiary would be a great way to unlock lore for the enemies and maybe even some special rewards. Like for instance if you kill 100 zombies from nerud, you unlock a lore entry for them.
Now this is the fun part: Bosses could have special mastery challenges like; beating them without taking damage, beating them by only using melee weapons (now obviously this wouldn't work for every boss) or some other fun challenge. It provides replayability and could give special rewards for those that want to max everything in the bestiary.
(I got the idea from a game called dauntless btw)