Any country can do that. It is also the only way for cultural-union countries to change their culture. Cultural union countries are France, Great Britain, Scandinavia, Spain, Italy etc.
Russia? Low stability cost?
You can tell by the lack of icon in the corner that the HRE has been disbanded. Regardless, I think it isn't possible to form it as Germany.
Challenge: guess the nation I'm playing as (latest 5.2 beta). Hint: it's not GB.
So as of 1680 I have achieved my goal - the world has been reduced to my one-province Netherlands and its 204 vassals and 7 unions. Huzzah!
Stuff