To have them progress through each patch from launch all the way to 1.12 in the original time frames. Including undocumented changes (of which there were HUNDREDS). Bring all the old tactics, talents, clever use of game mechanics and everything back as it was for a time. And slowly roll out each patch, each update each hotfix in chronological order.
Would be funny to see absorb shields work on falling damage, 15 man UStrat runs, Shamans being gods when wearing Invulnerable Mail, Rogues being gods with weapon swap macros for instant attacks, Warriors dealing 90% of peoples life with slow weapons before normalization, Mages AoE leveling in Mara on slimes, Warlocks being free kills until a re-work and updates made them viable all of a sudden, a period of time without Diminishing Returns in pvp, Plus.... I want to spec Lacerate for the pure nonsense of it.