Me along with a few other people know how to decompile the Unity client already, and most of us are knowledgable in the language, so writing a modded client shouldn't be too hard unless DECA decides to add some trickery to keep modified clients from being able to run on their servers. In which case we'll have to find another solution down the line.
But yes, modded clients most certainly are possible and will happen. Question just comes with who will do it first and who will do it better so more people can adopt it.