I think what I picked #3 is the most likely to occur aside from Mexico being there and the EU instead of France and the UK separately.

I'd prefer that it be by GDP, ability to send expeditionary forces (the SC members would also be the enforcers ), as well as be a country that allows at least elections for the head of the government (in order to let the UK keep a seat) with free open, multi-party elections (I'd hope that by 2050 the PRC meets that requirement).

Once again I can't really see France or the UK giving up their seats to a EU power. The UK for reasons of past greatness and idea that they like being not entirely hemmed into the EU. As for France, I couldn't imagine it giving up it's seat to an EU that could very well be dominated by the Germans.

China is the only sitting permanent member that would want power by population so I doubt the others would give in to that.