At the moment, I have the Achaeans in the mod. However, a comment by an EB team member sparked me to look into things again (something along the lines of the Achaeans not being considered because they were a very minor power, unlike the Aetolians, who had just saved Greece by defeating the Galatai). Here is the result of my research on the Achaeans:
-In 264 BC, it should be a group of 12 Achaian cities, among them Patrai, Aigion and Dyme.
-In 251 BC, Aratus of Sicyon brought Sicyon into the league. Sicyon was not an Achaian city, because it was Dorian, but they took the name Achaian thereafter. At this time, "the Achaeans [...] had neither brilliant repute nor great strength." [Plut. Arat. 9.4]
-In 243 BC, Corinth was taken [Plut. Arat. 16.2]
-It was only after that, in 235 BC, that Megalopolis under Lydiades became part of the Achaean League. [Plut. Arat. 30.1 and following]
On the other hand, the Aitolians sent their largest muster of troops ever to the Battle of Thermopylai against the Galatai. This amounted to about 7,800 troops. Adding to that the Locrians and Phocians, which would be in Aitolian territory, you get almost 10,000. [Paus. 10.20]
The ability to raise such an army - comparable in size to that of the Boeotians, who went into decline - and the Athenians shows that they had some population. They also maintained their independence for a long time, until 189 BC when it was forced to become incorporated into Rome.
However, they are noted to have had limited manpower and wealth.
Neither of these are exactly ideal. The other options are Athens, which was in sharp decline (so no!) and Sparta (which was also in decline and has no more right to be there than any of the others). So, erm... I'm stuck. Ideas, anyone?
I'm inclined to keep it as the Achaians, now that I have assessed it... it will be easiest