Buliding a real time "API Builder"

For a project I was thinking of making a real time api builder. The application should be easy to deploy and would allow users to easily create API endpoints and specify what data needs to transmitted via these endpoints. The API builder would also allows users use whatever application to consume the API, whether it be web, desktop or mobile. In order to make the API easily consumable, I was thinking about using a preexisting messaging protocol such as XMPP or MQTT. The problem is that these messaging protocols require a broker or server to sit between clients. I have not decided whether the API builder should be a web application (I'm more leaning towards this) or a desktop application. I know that there libraries and frameworks for web such as SocketIO and Redis that I can use to implement such a task but then I would have to develop my own messaging protocol instead of using a standard one. What would be the best option for developing such an application? Should I try build my own messaging server/broker for XMPP or MQTT? Should I make my own messaging protocol?