You've always been able to do this. Whether its useful to your clients has always been the problem.

In a practical sense you likely wouldn't like the alternatives, because for most people's usage of the internet there's exactly one authority which matters: the local government, and it's legal system - i.e. most of my necessary use of TLS is for ecommerce. Which means the ultimate authority is "are you a trusted business entity in the local jurisdiction?"

Very few people would have any reason to ever expand the definition beyond this, and less would have the knowledge to do so safely even if we provided the interfaces - i.e. no one knows what safety numbers in Signal mean, if I can even get them to use Signal.