yes, it would be cool. It would enable all sorts of nations being available to be played as in the game without nations like Romans squaring off against the Italians, the Romans are the direct predecessors of Italians, so in fact it would not make sense to have them both available at the same time, but one of them being available per game makes a lot of sense to me.
Just avoid stuff like this example, here are some other ones:
Group of playable nations: again, none of these can play against any of the other group member in any single game.
FRANCE, GAULS(GALLS), FRANKS(HOLY ROMAN EMPIRE), NORMANDY(OR BURGUNDY)
In this example French can't only face off against these 3 nations, which makes perfect sense 'cause Galls are ancestors of Franks who in turn are ancestors of the French and Normandy or Burgundy are closely related(could share a lot of the same city names this way)
another group:
GERMANY, PRUSSIA, GOTHS, SAXONS
all closely related, although you could fit in Prussia and Germany as separate nations.
other examples:
SCANDINAVIA(as a one nation), SWEDEN, DENMARK, NORWAY
GREECE, BYZANTIUM, MACEDONIA, MINOANS
GREAT BRITAIN, ENGLAND- only got 2 nations here, 2 others would have to be added, ones which never had contact with these nations, so that we could recreate history during our games, for example: CARTHAGE AND PHOENICIA
this way they could divide all nations, add several new nations, just keep the closely related to each other together as well as those who never came into contact with each other throughout history together as well(if need be)
example of a group where nations never faced each other in history(for the earth scenarios especially important)
CHINA, INCA, MOROCCO, SUMERIA
The groups could be made of 3 nations for a total of 60 tribes together, divided into 20 groups-so that instead of playing only against 14 opponents tops, you could play against 19tribes
details would have to be worked out, but this way Italy and Rome, France and Gauls(or Franks) can all exist in the game.