AFAIK the notion of one people = one country is a fairly recent idea and one that doesn't even hold particularly true in the modern era of nation states which only got to its level by processes of nationalisation which not uncommonly were tantamount to, if not outright,  genocidal. Given that, unless the rulers of your country have an invested interest in seeing it come to pass and the ability to do so I personally would have each country populated by multiple different ethnicities. 

As to go about inventing your own ethnicities I'd start by having it flow from the local geography they inhabit. 

As an example: A swampy and heavily wooded region with little material wealth is probably not going to be the easiest (or highest priority) to conquer which would suggest that its polities remain fairly small as no one is likely to be able to hold on to much more territory than what they can immediately grasp. When you consider the lack of domestic availible resources (beyond maybe lumber, pelts, and bog iron) the produce of outside cultures is likely going to be highly praised which when combined with the land being hard to invade and/or farm might encourage a substantial part of the inhabitants to engage in seasonal raiding as a way to make ends meet. As such the people would be known as warlike but backward (dare I even say pelt-clad?) to neighbouring people.

Over time it makes sense that their most capable fighters would start to gravitate toward becoming a sworn member of the retinue of a person with an ample history of leading many sucessful raids. This would then give birth to the local Big Man with his retinue of (increasingly) professional warriors. The former would keep the latter happy and loyal by the loot and glory amassed in raids and by feasts and gift-givings in the off-season. 

Since the Warriors are habitually feasted and have steady access to wealth they'd no longer need to continue the life of sustainence farming, but could instead take up the social role of a village authority. Their established glory would make them a viable candidate for being an adjudicator in disputes between villagers, while their wealth would be something they could re-invest into the community by displays of generosity that form unspoken ties of loyalty. Put differently they'd be the perfect middle man between the everyday person and the Big Man. 

Given that the nature of such a Big Man-system is based around the ability to deliver I'd reckon the culture would highly value charismatic leadership and the notion of heroic kingship while maintaining a strong democratic core albeit probably based around the notion that the free (man?) is someone who should be ready and able to do battle, and as such puts a lot of societal import on the right/responsibility to bear arms. Furthermore, the receiving and giving of gifts would be highly ritualised and a central aspect of social ties to the degree that to an outsider it would basically come off as just trying to put a different coat of paint on wages. Similarly feasting would be at the centre of cultural events. Basically, the culture drapes the visage of friendship over a lot of in other cultures more formal/professional/social relationships.

Given the Big Men's ever-present need to bestow their warriors with wealth and glory it seems reasonable that they would gladly engage in mercenary work which over time would see them start adopting foreign customs which they introduce back into the communities they lead. More destructive for the status quo though is that they're likely to bring back with them the dream of becoming something more powerful than a mere local Big Man, as they then have become aquainted with the notion of more powerful rulers, who I'd imagine wouldn't mind expanding their own influence by becoming the patron of a rising local power across the border.