Maximum realism would be great but my concerns are that too complex a system could needlessly inflate render times, even though stuff could be disabled to increase render time, i don't want to create a system whereby it's neccessary to turn stuff off for every render, especially since this is to be used as a sort of foundation upon which to build. If your foundation alone takes a day to render, imagine how bad it would be once you actually did something with it. So, there is a threshold. That said, I don't really know exactly where that threshold would be, and it would be different for different users and different machine specs.
I would want something relatively simplistic, but also capable of simulating complex realistic nature systems acceptably enough to make it universally useful. Impossible? Not sure yet. I think we can get pretty close though.
The way I intended to approach this is to start with the bare essentials. Implement the most basic system as a goal, and do it really well, such as the hydrosphere, and then call that a version. Then, the next version includes continents, and so forth. I think it might be really interesting to do this as a community project. If someone out there is really good at making a full-planet hydrosphere, then they should do that and submit it. If someone can do really realistic continents, then they do that, and so forth. As long as the contributors work with the understanding of keeping render times manageable, and there is someone to oversee the entire project, it could probably be done quite rapidly and the results would be quite good.
The big problem with any project like that is management. When enough people are working on something, it becomes a seperate job entirely just to make sure everyone is on the same page and the work being done doesn't end up clashing with the work that needs to be done. And so the question becomes, is a multi-contributor approach neccessary?
This is why I started the thread in the first place; to see if anyone else had already done some work of thier own that could help. But there is definately room for discussion, and I am starting to see that this is a project worth doing, which will help to motivate me.