Disclaimer: I studied Etruscan/Rasna history but it is very rusty and dated.
There doesn't seem to be a 'league' alliance type that properly represents them. The Rasna at the game start were a confederation of 12/13/15 independent-ish city-states (depending), but are generally shown on maps as a united polity when the truth was more in between. Their conquests would be sponsored by one or more city states, and not the league as a whole. The closest more modern analogue would be the Swiss confederacy, I think.
Failing getting a confederation alliance type into the game, the Rasna being tribal 'works', in the sense that if they had a lot to overcome in order to properly centralize. Another issue they faced was a potentially self-fulfilling prophecy - the Rasna believed that they were given ten? (I thought it was eight, but Wiki says ten...) saecula for the total lifespan of their civilization. In any case, around the game's start they believed their decline was inevitable.
Of course, being tribal doesn't work in the sense they were certainly one of the most civilized powers of the era. We use their alphabet, and the word the game uses for them literally refers to what the Romans knew them for - building towers.