How to tackle designing abstractions for an api that sends and receives unstructured data?

I am currently working with an api that basically acts as an interface between the developer and many different databases. The database exposes multiple sources of data and different types of data .The api only exposes one endpoint with the ability to receive json describing the data you want back. How would you go about designing the abstraction to be used in code. Would you create a class to represent the client and one to represent this query, and then the query is given to the client....Or maybe just write a method that takes json and returns whatever is given back?