The problem with Roman-Germanic Wars, it would be too similar to CiG. Grinding through near endless germanic hordes as Roman. The same thing would be with a conquest of Britannia. Grinding through endless hordes of Britons.
An Alexander-DLC is the only thing what could bring something new (Mauryan-Indian Faction). Or you make a Trajan Wars-DLC, but Dacians, Nabateans and Parthians are already ingame.
Selling Point?