Sony's doing it for EQ2.
https://player-studio.soe.com/
And Neverwinter also has a similar system.
I do hope the game is at least as moddable as WoW, that'd be very cool to see. WoW had a ton of useful mods and a ton of role-play friendly and immersion mods that worked pretty well. http://www.curse.com/addons/wow - good lord, 5,641 mods...
With TES' history of being so moddable I hope they would make some concessions to this end. Having both the WoW moddability, which was most cosmetic, and UI-based utility mods, AND the content moddability that both Neverwinter and SOE are touting would IMO be a huge boon for the game. I know Neverwinter will have a rating system so that people can "test" the content and review it. Of course there are limitations in terms of game balance and lore, so you can't make some crazy nuke-launcher weapon that you have to fight Dr Robotnik to get but within a set of limitations, I still think it could be very rewarding indeed.