I'm sure a lot of people would love to create their own Ages. Cyan, I think you should release the SDK programs you used for Uru to the community. Many developers have done this, and the fan-created content expanded the games' replay values phenomenally.
There are two main reasons why you may not want to:
1. It's not user friendly: Well, neither are most SDKs. If that's the case, then only the hardcore programmers in the community will create new Ages, and judging by the amount of quality mods made for a lot of games, there are a great number of them. You don't have to provide any support for it at all. Just type up a short Readme file explaining the basic features and we'll figure out the rest.
2. You want to maintain realism: I understand there's a big roleplaying aspect involved in designing Uru. I'm not sure if D'ni is real. But the same way Call of Duty is a game version of World War II, Uru is a game version of D'ni. You can use creative license. So what if people create Ages that aren't real? I'm pretty sure that Rise of Nations isn't depicting the Mayans taking over the world as a real thing, either.
So, please, release the Age creator programs. Or at least give us a way to create our own Relto pages and Avatar clothes?