Koinon Hellenon possible areas of migration:

Iberia - preferably to Emporion as it allows a slightly larger unit rooster, including the Greek Classical Hoplite and the Greek Medium Cavalry. A quick conquest of Vellika will give you armour-piercing Asturian Axemen, a worthy backbone of your army. Avoid attacking Numantia as it will provoce the Lusotanians.

Gaul - centered around Massalia, a celto-hellenic empire in Gaul can be very challenging. Thorakitai, Massalians Hoplites and Celto-Hellenic Infantry (which can be recruited in every historical celtic area) will be your backbone. Massalia will also provide you with artillery and heavy celtic cavalry, the Brihentin.

Britain - As Gaul, but even farther away! Conquering Ictis and Camulossade will give you Celtic-Hellenic Infantry, but also puts you in greater risk of being attacked by Casse. Ireland is basically useless, unless you like armies without cavalry, consisting of only fyra types of units.

Crimea - It has already been said, this offers one of the most interesting mixes of armies, hoplites and horse archers. The unit rooster for Chersonesos is impressive, and with its twin city, Pantikapaion, you will have a powerful army of your best hoplites, best heavy cavalry, artillery, great archers and great horse archers.

Krete & Kyrenaia - Just south of Greece, in North Africa, lies the city of Kyrene. There you can recruit Greek Phalanxes to ward off the Ptolemaioi and the carthaginians. Thorakitai and Light Hoplites will also be available to you. With the island of Krete also under your command, you will have elite hoplites and one of the best archer units in the game as well.

Baktria - If you really want to disturb the map, walk (or move_character) your armies to Baktria. Baktria provides horse archers, thorakitai, artillery and various cavalry to annoy Baktria, Pahvla and Saka Rauka with. Maybe a trip to India too?

Conclusion: I will start a couple of migration campaigns right now...