I think I'd want areas that could be easily compacted with other content, like how Ashfall is COOL, but is primarily where people go to do Dragonslayer/Akri.
When Ninja/Pirate drops, I think I'd want to go back to Shonuff. Nothing funnier than watching level 5's who try to sequence-break get slaughtered by lava globs, so why not Monkees?
If and when the game gets to the point where PvP is put in and classes begin to shift towards being more complex (Ninja on PTR is INSANE right now), I think it'd be nice to have a Dummy Room, possibly as an extension of said PvP hub.
As far as just cool stuff? Void, definitely, and maybe a raid boss that's just a normal (or corrupted/antagonist-piloted/etc) Newbatron. Maybe that and a possible Falconreach could coincide with DF/MQ "Loyalty-Bonuses" how Guardians get stuff in game?