I'm still enjoying GW2, but... (Tier related)
Someone I know had a good idea: have an item, or buff, or something, that makes you substantially less powerful. Like, it actually has negative stats (or something). But the tradeoff is that rewards are scaled upwards. For instance you talk to an NPC that sends your effective level 5 levels below whatever the zone is scaled to. Then you go to a 15-25 level zone and its very hard, as though you were trying to do it on a lv10 character, but the rewards are bumped upwards.
It would be like a version of GW1 hard mode. The people who want to do difficult content could do that, the people who just want to farm Cursed Shore can do that. It would also give an incentive for lv80s to revisit old zones.
Basically I think there are several ways to introduce (optional) more difficult content without having to make entire new zones or dungeons.