The reason YAGNI applies to code is that the cost of change is low. With good, well refactored code adding a feature later is normally cheap. This is different from say construction.
In the case of protocols, adding change later is usually not cheap. Old versions break, it can lead to communication failures, and an N^2 testing matrix as you have to test every version against every other version. Compare this with single codebases where new versions only have to work with themselves.
So in your case, for the protocol design, I wouldn't recommend YAGNI.
