Looks like it could be modelled by having different tech trees. The Celts could be limited on the civic side, for example, or make the civic side more expensive for them.

A bigger problem in gameplay terms is a cultural one. The game we have now is very much a product of modern political ideas retrojected onto ancient times, so that the different cultures are modelled as nation states, and military conquest is accomplished by occupation, and so on. This doesn't allow something like the sack of Rome you mentioned, nor of the Germanic migration/invasion toward the end of the Western empire.