1st option is my favorite..
Although it will need a lot of work more than the 2nd option it will be more educating and more chalenging..
It will need two separate roman rosters (if the ending date will be after Yarmuk),it will need Avars and other factions but...
The player will find him self in a living hell...This situation will help him to understand better what chalenges the Empire had to go on through to survive and the real value of the real Heracleius as a leader ...
Revolts,desertions and multi direction invasions or rebelions will help the player to live the despaired situation of that times...
Afcourse the value of the 2nd option is still great...
Again the mod will need 2 diferend Roman rosters espesially after the defeat in Yarmuk!
But this way the player will foully undestand the reasons of the reform of the roman armies to those for 700-900 ones....