Thanks for the feedback.
Today we've released the Web Service integration with Nintex Gateway that will allow you to create workflows that can access on-premises web services.
These calls are (unfortunatly) not returning structured data as we wouldn't know the definition of this structured data on the customer's side.
This means that this new integration is not available as a data source and thus would not solve this request.
We added the ability for custom xtensions via Gateway(https://help.nintex.com/en-US/xtensions/03_Advanced/06_OnPremises/PRC_04Gateway.htm?TocPath=Connect%20and%20authenticate%7C_____8), this would be the recommendation when an API is not accessible externally.
Thanks for your feedback! Is the internal API on your local servers? We have done a proof of concept and are looking at providing the gateway functionality for custom connectors in the coming months.
In regards to bringing back customer information and displaying it in multiple controls, we are working on this at the moment in a feature we are calling "Data Source Variables". We are looking to have something ready late 2019/early 2020.
Cheers,
Rob