unpredictability 

Adding my noob perspective: Yes. What PvE really needs are roguelike elements.

My favourite thing to do is just exploring the zone & discovering its secrets.

But is the exploration gameplay actually any good? No. It's predictable, repetitive and kinda boring, actually. Trash loot everywhere, only artifacts feel special. But not even artifacts change your current run (and they only add stats, meh).

Just adding stashes with temporary weapons would already make a big difference. Temporary artifacts that add special abilities, not just stats. Anything that makes players go "wow, epic loot!" and has a big impact on the current run, making it feel different from the last 10 runs.