If you're looking at Total War mods, I'm obliged to stan for Europa Barbarorum rather than Rome: Total Realism. EB was the granddaddy and has always been a more extensive and, in my opinion, better-put-together and better-researched mod than RTR. As hard as it is to believe, EB both gets the history right and makes the gameplay significantly more fun than in the vanilla game.
It has a sequel, though: Europa Barbarorum II for M2TW, which is more detailed and includes the evolution of government systems. For example:

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