Custom Server Side API?

Is there a way to create a custom api that can publish data to a Plugin registered on the server side? Ideally this would be something that I can talk to via javascript on the client. 

I am aware of the Azure integration, but from my understanding it only works with CRM data. I would like to be able to send custom data that is not just entity data.

Essentially I want to get as close as I can to writing code server side that handles a http request. 

I am on a Online instance.