I had a really good idea (I think), probably not for 0.8 yet but for some future release. I don't know if this is possible with 1.5 or if the EB team would consider it.
With the extra faction slot make a second Roman faction that does not exist at the start of the game (that is, if 1.5 allows emerging factions). I don't know what you would call it but it would represent the Republicans. Each Roman general would have either an Imperial or Republican trait. This trait could affect which historical ancillaries the general would get. After the Marian reforms have taken place and a general with the "imperial" trait wins so many battles or receives a triumph or something, than the civil war starts. All of the generals with the imperial trait stay under the control of the player and those with the republican trait join the second Roman emerging faction. After the player defeats the Republican faction, the Imperial reforms take place and the faction leader takes the title "imperator."
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