Arriving in character as 'the Honoree', Joe proceeded to answer selected questions from the storm that followed his appearance, mostly in clipped Rikti dialect. An in-the-flesh presentation at PAX filled in some additional information. We were pleased to see that missions will indeed be more sophisticated than just radio missions with new text. Players will be 'able to customize existing heroes, villains and enemy groups with a dozen or so options, if not more at launch.' It will also be possible to include Archvillains and Heroes, though some of this content might need to be unlocked.
Mission architects will work with basic mission elements, like Defeat Big Boss or Click Item, but it will also be possible to have 'escort NPC to location' features, as well as embattled allies who you can free and bring with you on the fight. Multiple factions can be included in the mission, and the ability to set different aggro settings means that mission architects will be able to rig up fights that are already ongoing when the players encounter them. Boss fights can be made characterful, too, with customizable dialog, behavioral options (such as fleeing when their health drops), and the running banter as they lose health.
The system is cross-game, cross-faction and cross-server. This presumably means that Hero/Villain co-op missions are a possibility.
Not all of the content will be available out of the gate. Some, and they're still deciding what, will be locked. Getting one of your homebaked missions picked as 'Dev's Choice' will be the way to unlock this. We can see problems with this, as rating wars seem extremely likely, and no doubt there will be many 'check out my mission!' forum posts going up.
While a few players are grumping that they don't want to be unpaid content writers, others are already planning out their missions and arcs. We still don't know if Dev's Choice missions count as canon, but a comment by Joe Morrissey confirming that the players should write the Coralax story themselves would seem to suggest so. Players might not have statues of their characters going up in Atlas Park just yet, but they just might be able to help extend the world they've been playing in.