The Aetolian League (Aetolia in-game) is one of the most interesting Greek states out there. They have the misfortune that their heyday was really the period ~280-220 BC that is also one of the worst documented periods in Greek history (Diodorus's chronicle and other sources give us a pretty decent history down to the Battle of Ipsus in 302 BC, and Polybius's history doesn't really take off in great detail until 220 BC, really starting right after they have been defeated by Philip V of Macedon). It doesn't help that Polybius was an Achaean (the rivals of the Aetolians), and so is basically incapable of describing them without harshly criticizing them.
We know they had some sort of representative league structure (like the much-better documented Achaean League) which allowed various city-states to join on equal terms with their founders (regardless of ethnic origin, which is unusual), and that they were able to expand from their Aetolian heartland to control most of central Greece at their height, being probably the most powerful of the Greek states aside from Macedonia itself and controlling Delphi (and its oracle) to legitimize their hegemony. They were also notorious for their raiding of other Greek states (although this comes mostly from Polybius, so the extent that their reputation was justified vs. Polybius's prejudice is still a matter of some debate).
In-game they are nowhere near as strong, but are still one of the larger semi-generic Greek states, so provide some opportunities there (they get the Greek missions from the Magna Graecia DLC, but none of the unique Spartan/Syracusan/Athenian ones). Haven't played them this patch, so not sure how the levy system and the Diadochi/Rome changes affect their balance.
