Take a data repository that you know (or develop) and see if it has an API to query based on tags.
Can we unite different those different search endpoints into one consistent interface from which we could have a central interface/index that allows users to easily search "all" data repositories?
The big difference with something like OPTIMADE here is that we wouldn't have to define complex standards and ontologies. We just require "arbitrary" tags to be assigned and that is it.