The greeks because they have come to be seen as the guardians of western culture against the hordes of exotic easterners and their despotic rulers, while also selling their services as infantrymen all around the eastern mediterranean.
If we take eurocentric history as inspiriation for new versions of regions that seperates civilisation from barbarity who also had a strong mercenary tradition then the celtic and classic greek civilisations stand out as fitting bases for borderland territories. Seeing as the Border Princes wasn't fleshed out army-wise besides some mercenary units and their fluff wasn't too much expanded on (atleast not what I'm aware of) combined with the wish of the official 9th Age fluffers to move away from the GW IP I see no real reason for this setting to try to hold onto the same sort of borderland. A medieval greece without an emperor and the imperial state would be extremely different in a way that would make it close to unrecognizeable. That might be hard to do seeing as medieval Greece was an empire with tremendous focus on the imperial person. So I would model anything equivalent to Border Princes around medieval Greece with influence from classical Greece. I think the Border Princes / Tilia in WFB was a medieval society built on the ruins of ‘classical’ humanity.
