Western Roman Empire: Divided into vassals
Play as Western Roman Empire, but instead of a whole empire, it is divided into many vassals with Western Roman Empire as the master. They are like the western version of the Sassanid Empire; easy to play as, hard to fight against with endless stacks from their puppets.
Auxilliary system has been carried over from previous mod. Roster is virtually limitless. Just build the Auxiliary building in specific regions (example: Persian lands give you Sassanid units).
You can levy exotic units from your vassals based on their region. Africa gives you African Legios, Hispanic region gives you Hispanic Legio, etc etc.
You can even play as one of the vassals if you don’t want to control a large empire.