People may remember that I used to do this with EB1 a lot - take Epeiros (who are the most disposable faction, IMO) and turn them into someone else. I used them as Pergamon and the Bosporans in particular, though obviously neither of those is necessary in EBII. I also used them to play as Massalia and Kyrene, which was a lot of fun. You get a different native mix to add to your Hellenistic units, along with different tensions and dynamics in your new home.
This would never be a total conversion overhaul, I don't have the time or the historical resources to make a proper "new" faction from Epeiros, and I wouldn't want to divert EBII's team from the main task at hand. There'd be no new biographies or government buildings (Epeiros' are fairly generic anyway), but there'd be new local pools for government buildings, a different starting location and upgraded infrastructure in your new starting capital. I might even change the faction's colour scheme (though that won't impact units).
Would that interest anyone? Massalia, Kyrene or Syrakousai are the most obvious choices; none would ever be proper factions in EBII, making them good candidates for this.
EDIT: Here's a results. No need to migrate or do anything but fire up a campaign (I wouldn't recommend playing as anyone else in this game):
Epeiros-as-Massalia
Epeiros-as-Massalia v4.zip
EDIT: Updated to v4. Major changes:
Now aligned with EB2.2m (so all those changes present).
Corrected an error in the Native Admin/Supervised Native Admin that didn't allow them to be built in Gaul/Iberia/the islands.
Fixed error with the Greek colonists script.
Adjustment to ship recruitment
I won't be updating this zip any more, too much rework involved and it's not as viable as the other.
Epeiros-as-Syrakousai
Epeiros-as-Syrakousai v8.zip
EDIT: Updated to v8 - aligned to 2.2r
Corrected double-placement of Syrakousai...
Corrected elite ownership.
Some text edits so things are labelled correctly
Adjustment to ship recruitment