Then how can we represent trade agreements? Certainly these play a larger part in the global economy than do individual corporations. I've never really like civ's corporation system... it's way too limited.
And how can we justify only allowing 7 corporations in a world with 40 countries?
Good questions - have some good answers.
For starters, at game start, all nations will have existing diplomatic relations; hundreds of turns of 'our open border draw us closer together', ''you have supplied us with resources" etc.
This should be easy to code. If it isn't, then 'play' the game from about AD 1700 to create the links and then make the save game at 2009 the actual module.
Now, sure Corporations are powerful. But so are multiple other things. Wonders: Frex, Saudi Arabia has the The Masjid Al-Haram, (and should have permanent Open Borders with the rest of the Islamic world).
Great People: Many cities will have attached GPs, frex London would have several Great Merchants, Paris some great Artists.
Resources/City Improvements: Europe would be pretty much stripped bare of mineral resources, in part because the continent is so well developed. Nations that aren't well developed will have more resources.
Lush terrain - Indonesia would have none of the above, but is a major player because they will have so many Grassland/Floodplain tiles, allowing them to draft 3 units a turn for the entire game, and still be able to whip if they feel like it, or just have huge cities if they don't.
Empty space - the Steppes have been Russia's primary defence for centuries. Although empty and useless, they make a fine defence, especially with an army of mobile units. So Russia, Ukraine, Mongolia etc should all be built around being unassailable due to terrain.
Technologies: again not equal, and sometimes nations won't *want* higher techs, as they make units more expensive.
Another option, that may be harder to code:
Can some nations be playing a OCC while others aren't? Because it makes sense for Israel and Singapore to be playing a single city challenge: giving them the ability to put more World Wonders in Jerusalem/Singapore respectively. If so, then any small nation has this as a leveller.