Lately I've been posting about the Google Apps Script Content Service and how to use it as a 'serverless' Rest API service. In addition, I covered how to use the ScriptDB as a secure repository for oAuth credentials, and using the Content Service as a proxy for other APIs that need authentication.
Another thing I've been playing around with in recent posts is mashing up multiple APIs, for example doing some geocoding then passing the result of that off to another API.
So where are am I going with all these threads? Content Service, acting as a quasi server, allows us to either combine, or preprocess API calls. That means that we can create a "completely new REST API" by combining multiple APIs, doing the necessary preprocessing and translations in Google Apps Script.