For the idea to work, we'd need at least Britain, the USSR and the USA to all agree on the location, as they're the greatest power immediately following the end of WW2. I image Britain would be very unwilling to accept a location outside of Europe or North America, whilst the USSR would be against anywhere too far from their own borders (they'd want to have influence). The USA would probably be the most flexible of the three, but would no doubt lobby for somewehere they had influence (maybe Louth America? Liberia?). Honestly, I think people wouldn't want it to be in Germany, they'd see it as having just fought a ridiculously bloody war, just to declare the enemy capital now also capital of the world, even if it was technically taking land away from them. The idea probably wouldn't go down too well...
Basically, you need to find a location where Britain, the USSR and the USA all have a fairly equal degree of influence that isn't Germany. Otherwise, the other powers simply wont accept it. I imagine lots of small and mid level powers will be unhappy at first, but they'll probably come around. Or the world capital will just slowly end up being left to rot. I think somewhere in the Nordic countries or maybe Southern Europe (Northern Italy?) is probably the best bet.
EDIT:
The problem with putting the city in the USA is that the USSR won't accept it and probably end up building their own version back in Russia. Same problem with Canada, most likely.