Oh god the terminology.
Writing the Phantaro-Theocratic Accord was one of my favorite treaties ever (so bummed it broke), just because it was basically half just terminology rules about who could call their realm what. I had a blast writing it and was looking forward to seeing regular diplomatic feuds about terminology.
In my head I had the Macedonia/FYROM dispute especially, but was also thinking of N/S Korea and PR China/R China naming disputes. I guess the China one is actually most applicable, but the Macedonia/FYROM one is more fun.