Even with a build that isn’t completely maxed out or optimal, I think it’s a bit too easy to get to the max attunement rewards (level IV) on Kurast Undercity, through both rare and legendary tributes on T4.
I would like to see a fifth attunement level added, that can only be achieved by clearing every mob on every floor of the Kurast Undercity.
I think this would prove to be a fun challenge for top-tier/speedfarming builds, and another “level” to chase.