I recently worked like crazy to get the "Been there, done that" achievement and in the process I naturally saw all the unused areas of the game and their unfriendly residents (Black Shroud, ugh). It should go without saying that not having these areas be used for anything is a waste, especially since they probably won't exist much longer as they are now. So what's the cheapest, easiest way for SE to squeeze some content out of them (which this game still needs)?
Assign one big difficult NM to the dozen or so aetherial gates (Coerthas comes to mind) which are just useless right now and very inaccessible because of level50+ monsters(or 80+ lol). Simply lower the level of monsters in that area by 10-40 and have this NM move through the entire surrounding area, as a roaming NM. For extra easiness on the dev team just have this monster be a super big version of normal monsters or reuse unique models from other end-game events. Make them tough battles (though seeing how there a few useless mid-level gates, could always add easier ones) to distract high level players. You could go as far as adding a system/lore theme over it (another set of leves? Goblin NPCs which allow you to fight by trading beastcoins? Sponsored by the three factions? etc). Have these NMs drop good gear or crafting materials for unique gear, that's very, very important.
I have to imagine content like this would be very easy to implement.
Obviously gates which have found other uses, like Hollowed Ground, wouldn't be changed.