Hey,
I recently came on the idea of making an UML of nearly everything. (Represent everything as visual models).
The basic idea is to start a webpage, where every user can edit this UML. Of course there have to be some version controlling, validation etc.
The general idea is that every person can add new classes to this UML and hopefully the UML once gets very well designed and big. I believe this could be very useful for a lot of people. Imagine every real world (or even virtual) objects represented in an UML.
I don't think that I will start such a webpage for now. I hope some people get inspired by this idea and make it real.

What do you think of it?

Have a look at the topics of Ontologies and Reasoning. People are doing this for years. There has only been a partial success in smaller sectors like medicine.

But never fear! Try it but you should consider to use languages like OWL and RDF(S).

Since Wikipedia is the biggest community edited knowledge platform, I would suggest to bind in articles in wikipedia
as classes in your graph.

Wikipedia itself has already many relationsships between "things" hidden in its links.

Maybe you can create a tool to systematically link these
articles together into hirarchies and general classes.


For the articels in Wikipedia itself, you could propose
a special type of "link" to that graph.
Something like

Dog:
[REL:SUBCLASS:Mammal]

with an explanation of its purpose.