But their Unique Ability conflicsts with Diplo Victory
Their unique ability is peacefully conquering City States. But then again if you have to mention it liek somebody said earlier on, it's useful against countering-enemy votes (i.e Siam's alliance with Jersualem for example, Austria could conquer Jersualem)
I have a feeling that it will either not be a permanent process and will perhaps involve a large amount of added influence over the city. I do not expect this to tie the city to Austria indefinitely, but rather to make them better friends.
If it is somehow permanent, which I really doubt, then it will have a very limited re-use.
They aren't going to add something like this and not balance it well.