I think there may be a case for giving the Eleutheroi a proper faction slot.

In RTW the AI seems to prioritize Slave faction territories as targets. This leads for instance to the Romani neglecting the Epirotes and going off conquering to the Baltic coast.

If this is still the case in M2TW, then giving the Eleutheroi a faction slot should knock this behavior on the head. It might also make the Eleutheroi more active and interesting.

Another solution might be to add the middle european Boii as a faction.