Bring back your challenges from Netherworld Reign
Norm and nightmare are fine I think, maps included. The only issue I see is they are pretty redundant (drag boss to X spot, boss loses immune). The hell one is not even worth the attempt on HC last I checked, especially for it's rewards. It's a great concept though - it's not as if you can just slowly clear, exit when in danger, and continue on. I really like the respawn aspect of it. Not a fan of the invisible map myself, rather it just be one long path to the boss. Maybe put the boss in a separate room.
Your endurance challenge in NR was cool. Maybe you can draw some inspiration from Median or other mods in terms of ideas for specific challenges.
Also, this is more personal experience and preference: I enjoyed Aftermath's cow level. Basically just a big map with 6 sub bosses, whom each drop a specific quest item. Once you cube all 6 quest items, you get a weapon with a curse proc to clear the perma-immune cow king. One person attacks with the weapon, another does damage to the boss once the immunities are down (only a short time frame per proc). Pretty good quest-style concept that involved several bosses and teamwork. Maybe incorporate some type of multi-level quest type challenge for one of them?