Asiaine has the best idea her. Actually i've been watching Extra Credit and they did mention something akin to this in an article about "Power Creep", about how 'perpetual' games like MMOs have later content start eclipsing older content.

One thing that was mentione was 'incomparables', rewards are given that cannot be directly compared to eachother. For example, can you directly compare the value of Shield Bash and Second Wind as abilities? We can discuss this forever but you cannot simply look at the numbers in the ability and say one is better than the other. Same way, can you directly compare the Needle of the Luminary and the Gae Bolg? Incomparables.

Which is in fact a pretty neat solution for multiple difficulty settings and their rewards. Any one content COULD have Easy/Normal/Hard, and each of those difficulties have their own unique rewards, which cannot be compared to eachother. So theoretically everyone would have incentive to run every difficulty.

Of course, that has very, very high development cost. Which is definitely not something that the players care about, but...