I really like the mastery system for the world events, but I think they could be expanded on. For each event, if you complete the mini-objectives (kill all waves before time runs out, activate all switches, don't let NPC companions die, etc), you get a better reward. I think this could be expanded upon in the dungeons as well in order to promote gameplay balance and engagement.
For example, it is time optimal to not even finish most dungeons. Just enter, kill all the elites, reset dungeon, repeat. If the dungeon complete rewards were better, and scaled based on mini-objective completions, then there would be more reason to finish them. One thing in particular would be to add penalties for dying. I have a lot of respect for the Hardcore experience of Diablo, but you can replicate that to some extent in the Softcore mode by having greater punishments for dying. Right now, it only costs gold, so you can play sloppy and neglect survivability with little downside. If your dungeon clear chest got nerfed because you died, though, that would encourage you to pay attention more.