So the Four civs the last game were:
America
Turkey
Russia
Germany
If we break it into regions, this is what it is:
Americas:1
Middle East: 1
Europe: 2
That means Europe would be divided into two, but the Other powers go worry-free from any real danger.
Right now we have the civs planned out are:
England (Europe)
Japan (Asia)
Therefore, to allow each empire to grow without a major (Human) getting in the way, these are the avilable regions left:
Americas: Only America should even be considered for this region, all other civs are WAY too weak
Middle East: Persia, Arabs, Turks, (and with a popular mod), Byzantines. Any of them fit, but you need a mod to rule good 'ol Constanoplie, Like I said
Africa: Civs kinda weak there, but if someone has a mod to put in Zulus, then maybe there is an option. Mali is plauged with Barbarians and Egypt faces onsaulghts from almost every Middle East and Classical Europe power. Carthage could be acceptable, but could run into some problems with England.
India: Well,... India. Not the best empire, but Core area is well protected by nature and India could rule the southern part of Asia and Middle East.
Oceania?: Khmer could possibly, once spawn, settle into the Phillipins and make some sort of weird Oceania like nation. (I'm guessing on this one)
You could possibly add another Europe civ (Rome or Germany, maybe?) ad have UK find power in extra-Europe colonies, but most in N. America (Which has the best spot for colonies) will turn American.
Those are my suggestions.