Something I've thought would be cool was a concept I somewhat lifted off the alternatehistory.com discussion board - the so-called "Mosaic Earth". The basic concept is that countries/regions/societies from various timelines being transported to an empty world where they come into contact with one another.
This could create some very interesting possibilities; say a splinter Mongol kingdom occupying portions of France from a world where the Mongol Empire stretches from Atlantic to Pacific oceans meeting a surviving centralized Kievan Rus, or European provinces of a strong Byzantine Empire where Manzikert never happened encountering surprised Ottomans who expected to find an underpowered, dying Empire of OTL. Throw in the surviving Western Roman Empire (or few provinces thereof), the Caliphate of Rome, the Christian Arabia where Mohammed converted to Christianity instead of founding Islam, thriving pagan Vikings, and whatnot, and it could be one interesting mix, considering all cultures could be relatively equal in terms of playability, starting power, difficulty, and with very little predetermined history.
Something like this could be perfect for MP, and even in SP if the nations start with no knowledge of anything beyond their borders (but do get frequent random conquistadors and explorers), it could make for a reasonable game of exploration and colonization, considering parts of the world could be intentionally left uninhabited.
Some nations could start with a stronger tech than the others, but that could be compensated for by a different tech tree/more difficult start position/less access to COTs and/or resources.
What do you think?