[alganet]

Why does it have to be something non-trivial? Why do a lot of humans need to agree?

You can have an ontology that is used only by you. Maybe a 1000 people need to agree, and they would probably be on your payroll. It could be something trivial and already kind of decided, like movies metadata, etc. It's there just to power your internal systems, not for humanity to agree upon.

For popular use, it really comes down to the tooling. If I take this knowledge that I already have and write an ontology for it, what do I have to gain? Sadly, with the current state of tooling, you gain nothing.