So most veteran players would agree that there aren't much options outside of endgame to challenge oneself. The only way to really accomplish it is through challenges like Ironman or leveling without talents, whatever it might be.
What if there were a simple tool in the interface in which you could adjust your stats/damage/healing/armor on a scale from, say, 10%-100% of maximum balanced power (100% being the current, blizzard implemented spell power). The interface would work very similarly to the graphics/video menus.
The huge benefit I see to this type of technology would be a way to revitalize all old content by simply allowing players to make it as challenging as they'd like. You would never have to balance low levels again and it would add an infinite number of unique options for players to run with. Another huge benefit? You don't have to gimp anything else. You can use all of your spells, abilities, talents, etc. to their full advantage.
I'll give an example: a level 70 twink guild wants to raid ssc/tk again but feels way op'd
The entire raid group goes in with any gear they want (presumably way better than they need) and nerf themselves to whatever equivalent ilevel they want. Self-nerf heals by 75%, damage by 75%, and health by 30-50% and you're sitting right where you were 5 years ago with respect to character power. No raid tuning required anymore by blizzard!