I'm confused about the part where OP gave example of pre50 dungeon as interesting.
To be honest, most of the pre50 dungeons are really boring (almost fell asleep lots of times in it).
If the definition of good design is Qarn and AV, I'd rather do the current end game progression.
At least Coil is challenging to progress through. Qarn and AV ... It takes a lot to screw up in those dungeons.