Sure, you could go backward and wipe out old, lower-level enemies with nary an eyeblink, but once you've moved on, why should you have to go back? I understand the need for higher-level enemies to act as goads, spurring the player on by promising enticing rewards once you've reached the same status they possess, but once you've reached it, how is it any different than bumping along at the same level you've always been? What's the solution?
Well, perhaps multiple lower-level enemies could gang up on you at the same time. Singly, they're no match for you, but en masse they're an appropriate challenge. You'd definitely need to either group or devise a new strategy to deal with them all.
Another way to indicate might is by the old 'Presence Attack' concept made popular by the Champions roleplaying system. Upon introduction to a scene, your character had the opportunity to make a grand entrance, displaying his strength by destroying an object, or making a heroic speech intended to terrify the criminals at hand. This was called a Presence Attack; your character is so powerful that merely showing up stood a chance of causing your foes to flee or surrender. This could easily be a part of any combat system -- it's just math, after all. Any given opponent would be susceptible to terror at the mere sight of you, based on their underlying stats.
This audience is a learned and clever one; send in your thoughts on how to obviate the same old, same old leveling issue!