Developing multiplayer WITHOUT using UNet, Photon or other middleware?

I'm interested in developing a multiplayer experience that runs Unity instances on the clients but communicates with a server that is my own on the backend. To be clear, I'm looking to build a multiplayer experience that I can use even if I'm not only using Unity to build my game anymore. I'd like to be able to not have to start from scratch if I go to another development platform other than Unity. This is on the lines of ideas for architecture and tools to be used. Any suggestions would be of great help!

 

You could basically grab any c# networking library and build on top of it...