I will also for next round introduce two changes/limitations to which armies and units a defender can use, whether in a Roman Civil War battle or in a Foreign Invasion, aiming at favouring attacking provinces:
A defender controlling Italia (I) can only deploy the Praetorian Guard unit if the battle takes place in Italia (I).
A defender can only choose the ally of a certain province if the battle takes place in that province.
The additions to the rules are highlighted in 
green below, and the second post of this thread is updated accordingly:
Units
- The Praetorian Guard unit may only be deployed for battle by the player controlling Italia (I).
 
- Furthermore, the Praetorian Guard unit may only be deployed in battles outside of Italia (I) if the player controlling Italia (I) is the attacker.
 
- This means that a defender controlling Italia (I) can only deploy the Praetorian Guard unit if the battle takes place in Italia (I).
 
- However, in the event of a foreign invasion not even the player controlling Italia (I) may deploy any Praetorian Guard units.
 
- Subject to the above, all units in the Roman 24 BC – 196 AD army list are freely available to deploy (other than the standard in-game limitations on numbers, etc.).
 
Allies
- Allies may only be selected by players controlling certain provinces:
- Only the controllers of Germania (VIII) or Pannonia (III) may choose Germanic Foot Tribes 105 BC – 259 AD as allies.
 
- Only the controllers of Germania (VIII) or Dacia et Moesia (VI) may choose Sarmatian 25 – 375 AD as allies.
 
- Only the controller of Syria et Arabia (XV) may choose Arab 312 BC – 299 AD as allies.
 
- Only the controller of Cappadocia (XIV) may choose Armenian 331 BC – 252 AD as allies.
 
- However, a defender can only choose the ally of a certain province it controls if the battle takes place in that province.
- Example: a defender controlling Syria et Arabia (XV) can only choose Arab 312 BC – 299 AD as allies if the battle takes place in Syria et Arabia (XV).
 
 
- In these cases, it is always up to the player whether allies are in fact selected or not, and the player makes this decision for each individual battle.
 
 
- Notwithstanding the above, allies can never be selected in the event of a foreign invasion.
 
These changes become effective already in the next round (round 2).