Andy is absolutely right, that some thoughts and sketches should kick the process off and lead to a finalistion and solidifying of the concept.
In my mind, in order to avoid the Age becoming an eclectic mish-mash of bit and pieces, I see it as an Age based around the idea of a garden or gallery of elements that follow a consistent theme (think Eder Kemo). This would allow some the freedom to choose to contribute a single piece of art or sculpture that fits into the theme, or they could contribute a more fundamental piece of the terrain or structure such as railings, gates, plants or pathways. Perhaps two or three individuals could regularly share a .Blend file between them, adding the elements as sent to them by others, sizing and placing as necessary, bringing the whole lot together.
The key here is to make a small Age that is finishable without great pains, but is so stylised and well-thought out that it is complete in it's own right. That's why the thinking behind it probably outweighs the modelling, texturing and lighting. It should be clever, not complicated or unwieldy. Plus, we all want to progress with our own writing, so this should be something we can dip into or use as a break from our own projects.
Edit: Actually, Relto is the perfect example of size and complexity. So this is a ripe opportunity for someone with great ideas to chip in, whether they can actually build or not

-: D'eux :-