Well, you can use web api in Kentico 11 right now. You might as well create external Web API apps (using or not Kentico API there if you need it) and call it from Angular. One of the approached we've adapted is to create modules in Kentico to manage data in external DB and have a separate web api 2 app to serve it - so our angular running in kentico calls those external APIs to get a data from other DB. It really depends on you situation and what you are trying to achieve.