I think 'Grumblegrunt' has a fair point. 'petergriffin' too makes a valid observation. Clearly Russia's extreme size and large arsenal of nuclear weapons has no relevance in their being admitted.
China will ultimately head the league table.
India will be docked points for endemic corruption.
Brazil will face relegation because of the shambolic state of its infrastructure and organisations.
Dark horses include:
Malaysia and Indonesia - provided they don't start fighting each other and that the wilder elements of Islam are held in check.
Australia - size, but hampered by small population.
South Korea - hugely dynamic but held back by the lunacy that is North Korea.
Who is 'in' the G8 anyway? USA; Germany; France; Italy; Canada; Britain - err! umm!
I think the G8 has largely faded into obscurity anyway as a forum for regulating the global economy. At best, it's now just a place where the member states try to thrash out a common position to take to the G20.
I wouldn't write Brazil off entirely just now. They are improving their infrastructure leaps and bounds, mainly with the help of substantial Chinese and Arab FDI. India's problems with corruption are shared with a number of other countries of decent economic performance and I think it'll be other structural problems that determine the limits of its rise. Indonesia has been a real dark horse of late and is one I'd put my money on as a place to watch in future.