One of the things I enjoy about EQ2's epic quests is that it brings people back into older, otherwise unused zones in the game. I wish SOE would come up with more permanent reasons to revisit older content.
One good way to do that would be by introducing a new faction of historians or scholars who send people on daily missions for tokens, similar to the TSO scholars. Adventurers would go into special versions of Unrest, Poet's Palace, and other zones that don't receive many visitors anymore. The special zones would be mostly the same, except they would scale to the latest level cap and have new loot tables for the higher tiers.
Or maybe the adventurers might go into regular open zones, such as Sanctum of the Scaleborn, even though that means almost everything is gray, but along the way level-appropriate "ambushers" show up at various places while you try to complete your mission.
It takes time to create new loot tables for each additional tier, as well as itemizing the faction merchants, but I think it would be worth it. This game has a lot of content and its a shame for so much of it to go to waste because the majority of the playerbase has leveled past the point where those zones are viable. Or, in the case of some of the ROK instances, mudflation has progressed us to a point that it no longer makes much sense to go there.