It wouldn't be hard to build in tools to aid discoverability. A 'help' function that returns a list of features for example. That's pretty much what WSDL[1] tries to be, but that's really to hard to use well. If there were a much more flexible "natural language" interface to it then it'd make finding and consuming APIs both easier and more robust.
I'm just musing on the possibility really. I'm absolutely sure there'd be difficulties in the technical implementation, and obviously it'd be quite inefficient compared to a known and well-defined API, but if it could be made to work the advantages may outweigh those problems. Plus, implementing it for humans goes 90% of the way to implementing it for computers too, so it's almost zero-cost if you're considering a chat interface to your service. It's something to think about anyway.
[1] https://en.wikipedia.org/wiki/Web_Services_Description_Langu...