Part of the Problem is which Faction would the Republic be aligned with? The Rebel Alliance to Restore the REPUBLIC or the Empire which is what the republic became? and would the CIS be a 4th faction since they are rebelling against Big government, which is the Empire but the Rebels want to restore the "better" big government that they separated from in the first place, (they could maybe go Scum but they aren't really "criminal" either)
Resistance and FO have much clearer boundaries and are easier to align with existing factions which is why in my opinion Prequel era stuff is being slowly phased in from Rebels as early Civil War rather than 2 distinct sub factions
Edited by Gundog8324Clarity