If you haven't already, you should start learning how to test you application. Consult the documentation for the framework lot library (express, Koa, loopback, restify, etc) as any gernerators included with them might be able to scaffold tests for you.

After learning some basic testing, start adding custom methods / custom routes to your app's API. Most applications even though based on CRUD, provide methods that may preform logic outside basic CRUD operations.

Finally try integrating access controls (ACLs) and user authentication. ACLs define which users are allowed to preform operations on data within particular areas of you app.