As I mentioned before, I believe that these eight factions will be somehow based on the eight civilizations described by Samuel Huntington in "The Clash of Civilizations". They are: the Western (with Europe, Canada, the US, Australia, New Zealand etc, together), the Sinic (China, both Koreas, Taiwan, Singapore, Vietnam and maybe some Southeast asian nations affected by the chinese diaspora), the Islamic (Middle East, North Africa, Indonesia), the Hindu (India, Nepal, Bhutan), the Orthodox (Russia, Ukraine, Belarus, former Yugoslavia excepting Croatia and Eslovenia, Bulgaria, Romania, Greece), the Japanese (Japan only), and, with less certainty, the Latin American (Latin America and Caribbean, although it may be a part of the Western one), and the Sub-Saharan Africa (which doesn't exactly constitute any cultural unity).
With some needed changes, Japan could be united with the Sinic civ, the US (and maybe the Commomwealth) could split with the Western, leaving continental Europe by itself, the Hindu could be a part of a British Commomwealth along with Australia and New Zealand. This way, every faction will have at least 500~800 million people.
I begin with the assumption that there won't be an unnafiliated nation on Earth, with these eight factions covering the whole humanity. So it has to have cultural and political room for everybody.