I propose we begin negotiations by dividing up and exchanging established cities and then look at colony sites and territories second.
Here is my initial division/exchange of existing cities that will lead to a rough parity of numbers:
ATHENS/ETRUSCIA
Current cities: 26
After divisions/exchange: 54
+Sipontum from Corinth to Etruscia
+Andros & Tenos from Ionia to Athens
+Chalcis, Eretria, Cyme & Carystus from Ionia to Athens
+Tanagara & Thebes from Corinth to Athens
- Corinth from Athens to Corinth
+Nora & Aethalia from Barbarians to Etruscia
+Thirteen Barbarian Italian cities to Etruscia
+Two of the Southern Aegean Islands from Barbarians to Athens
+Eleusis, Epidaurus & Tiryns from Barbarians to Athens
SPARTANS/PHOENICIANS
Current cities: 39
After divisions/exchange: 51
+Petra, Enna, Morgantia & Motyca from Corinth to Phoenicia
+Zacynthus from Corinth to Sparta
+Patrae, Aegeum & Sicyon from Corinth to Sparta
-Miletus from Sparta to Ionia
+Cirta, Argos & Hierapytna from Barbarians to Phoenicians
+Two of the Southern Aegean Islands from Barbarians to Spartans
CORINTH/IONIA
Current cities: 68
After divisions/exchange: 53
Gain back two capital cities and give away 17 cities
That still leaves a few cities in the Balkans and Anatolia to decide about, but it's a starting point to agree on in principle or negotiate over.
The cities of Anatolia, the Northern Aegean and the Balkans are slightly more tricky to divide up as they do not fall into obvious spheres.
I will leave it here for now as there is no point looking at colony sites if we can't get a consensus on these major cities.