My thoughts:
Japan: think of it falling back into feudal times, controlled by a ghoul shogunate. Fights break out on the streets a la "Sukiyaki western Django" with katanas and guns in a new unpleasant post apocalipse. Instead of the brotherhood you have Ronin power armor wearers and fight a newly ultra national army.
Brazil: not very heavily nuked at all, it has turned into a jungle utopia on the surface and a free market nightmare right under that skin. Corporate dictators live in the main cities surrounded by cheap Favela labor and further into the jungle the tribes.
Russia: well you know... S.T.A.L.K.E.R.