I'm tempted to go with Hamlet Defense as a knee-jerk answer, but it wasn't as innovative as I'd like to remember it below the surface. Sure, I suppose there were different compositions to do it, but like most things, there was a universal composition that most people seemed to do to the point that it became as predictable and routine as any ARR+ trial.

I see a lot of other nostalgic things people are listing off, but honestly most cannot be feasibly be brought into the current game. Square peg into round hole, etc. (tempted to point out why on each point, but no).

I think my two picks are 1) the Path Companion (or literally anything that's not a dancing ostrichicken covered in armor) and 2) The 1.0 dungeons; with multiple paths and bite to them. The first one is pretty self- explanatory, but the more difficult and rewarding dungeons obviously couldn't replace the minimum-competency "Expert" dungeons the devs are comfortable churning out.

Things like having bonus chests when you kill the final boss for fulfilling different criteria (I'd eliminate the speed-run chest though) were a good touch, and encouraged things like taking dead-end/branching paths to open chests (to unlock the 'all dungeon chests opened' bonus chest). Since speed was a factor back then, many times the group would just send one experienced person they'd trust not to die to grab a chest on their own. Palace of the Dead/Heaven on High somewhat scratches this itch, and Bozja's 1st Castrum has shades of this, but I'd like to see these types of dungeons added as a new form of content- could even remap older dungeons and call them "unreal' dungeons.