I propose a Stack Exchange where only questions regarding theoretical aspects of Networking are covered. For instance, the theory behind a specific protocol, the idea/intention behind a certain implementation or some imaginary protocols. Most of these type of questions fall either in off-topic or the gray region between on and off-topic questions of Network Engineering and Server Fault.
Now why won't Network Engineering and Server Fault suffice for this purpose?
"Network Engineering Stack Exchange is for asking questions about professionally managed networks in a business environment." Here, the keyword is business environment. Implying by default that it will focus upon the usage of protocols currently being used in practice. However if an individual wishes to take a deep dive into the theory of that protocol, then they will often find themselves going off-topic.
"Server Fault is for questions about managing information technology systems in a business environment". Here also, the keyword is business environment. And moreover this is where issues regarding networking S/W & H/W management come out. Implying this place is way too practical for theoretical questions to appear. Forget about the deep dives.
Hence, I believe that we need a Stack Exchange where the questions regarding theory of Networking will bloom. It will not only give a place to the off-topic questions present in Network Engineering and Server Fault websites, but it will also allow people to enrich and enhance their theories with the help of the experts.