Ehm? No - that's not going to happen in an MMO of this nature.
Yeah, thinking about further, it might never happen.
It makes selling add-ons harder. So it might be no-go. But other Bethesda games also had add-ons, and yet they gave awesome modding tools.
Of course I'm not talking about modding the game mechanics, but allowing to create special areas, with the help of instances.
The only real problem is that it gives some players a way to cheat the game balance. What prevents you from creating a special area full of treasure, and then looting it?
A simple way to fix that problem would be that uploading a special area to online mod lists would require you to pay for your area. Say, you create a dungeon which has 1000gp worth of treasure - Well, that treasure is out of your pocket.
People who create mods are experienced players anyway, so losing that 1000gp might be worth the investment to them. They have a signature dungeon - it cost them money.
Another way to cheat would be creating easy XP. Say a high-level monster with convenient super-traps around it, allowing for easy kill.
Again, I think the "dungeonmaster" should lose the XP that a player can gain from his mod. A high level player can afford to lose 1000xp for creating 10 standard orcs, three tough orcs, and one leader orc.
That kind of approach would fix balance and cheating issues.
If you're creating an elder dragon with your modding team, you can divide the loss of experience between each member of the modding team.
Anyway, that's probably not something the gamemakers are interested in doing, so we probably never see any mods.
...And besides, I just realized it's stilly easy to cheat the system, when you work as a team. One guy creates a loot room, loses money. But 9 friends enter the room one by one, each collects the money, so the team overall has 9 times more money than what was spent.
The mods would need to be monitored before allowing for publication, by a moderator team. That would be the only way to do it. But it would be a too complex system to monitor, really.