You can instantly see where the additional development time is going. Players will now be able to 'customize enemies, rescuees, allies and contacts in what they are calling the Critter Creator.' In other words, yes, players will be able to put their own heroes (and villains) in the missions. Another key fact is how the rewards will work: there will be no mission complete XP and nothing in a player-generated mission will count towards badges, and instead of regular drops players will earn 'skeeball tickets', which will apparently work similarly to the Merit system.
All the maps and characters currently in the game can be used, which may mean that the original plan of 'unlockable content' for having a mission tagged as Dev's Choice has been dropped. Players will be able to rescue Statesman in the first room if they want to, but allies will not act like pets for the purposes of XP calculation; enemies they defeat will detract from your own XP. There's also a new option, a sort of 'hostile hostage' scenario, in which you can rescue someone who then attempts to beat you up all the way to the exit!
The venue for player-created missions will be a holodeck-like feature within a building resembling an arcade, which rather emphasises that these missions are not going to be canon, and are construed as simulations only within the game context. It all sounds fascinating, and we're grateful both to the developers for giving it and to Quilcene for putting forth the information.