Collaborative, Hierarchical Organization of Information?

I think it would be cool if we have some web tool that allows easy-to-use, collaborative hierarchical organization of information. Let me explain.

For my Acara Challenge class, there are 5 groups of 4 that are currently doing background research for the water challenge (writing a business plan that tackles an issue related to unclean water, sanitation, or hygiene in India; we get venture capital if we do well!). I thought instead of each group doing their own research, we should all collaborate together and practice economies of scale. So I thought about doing a Wiki. But as I thought about it more, Wikis are more suitable to catalog huge lists of random information (it's primary function is a searchable database of terms). Wikis are not good for allowing one to see the big picture, to organize information in a conceptual way that allows one to better grasp the underlying logic.

What is needed hence is not a searchable database of terms, but a tree-like structure. One can simply look at the overall structure to get the big picture, and if one wants more details in a specific point, one can expand a subtree to learn more. It is also conceivable that one may want to complicate this tree structure, with referral, similarity, and other types of relationships as well as parent-child relationships.

So I tried to do something like this using Google site. Here's the result (this drained many many hours of my time...): http://sites.google.com/site/cleanwaterforindia/. However, Google site is not that easy to use, and this approach does not seem scalable.

Beyond my water project, I see this broader need for Wikipedia to better hold such hierarchical, logical relationships. Right now the pages are just huge blocks of text, and grasping at the bigger picture can be difficult. So in the future, if anyone is interested, this may be a good idea for a non-profit tech. startup. :)