I had another idea to suggest to CA regarding the scattered and divided tribe cutlures ...
like Celts , Iberians , berbers , Germans , Britons , Sarmatians etc ...
DO no Treat them as one faction :
1) Allow multiple subfactions with minor expansion route , limit the expansions to usually sacks and raids , or interfactional expansion , but historically no Super porwer shoudl rise unless ... read after...
2 ) Give many many different subcultural factions , call them minor minor perhaps , so to have a good represetnation of for example Gaul into different subtribes all pertinent to one single Gaul Culture , briton , german etc.
3) enstablish an intercultural relation between the minor factions with semithreaties allowing alliances and enemitudes between them but still relatively different from an alien culture one .
4 ) Under a threat that affects more than one bordering tribe and menaces the whole culture of the area the tribes should unite and form a Rised army of tribes united under one flag or one carismatic leader to face the incoming thread ...
5) those kind of armies or temporary alliances can be very fragile but still strong for as long as the menace is present ...
So for making a couple of examples look at Arminius or Vercingetorix .
6) mass migrations under event trigger
For example ... An event like flood in central norther rhine regions could cause the aggressivity of a particular x tribe there to increase because starting a migration toward other lands or surrounding ones .... If it attax the neightbouring german tribe those may clash the looser may not necessarily be annihilated but eventually cold take a whle packed army move (with women ,childrens etc ) and seek new home... Attaking eventually roman borders ....
So events could play also a major role in what the evolution of politics and diplomacy could be...
7)
Neutral zones and nomadic areas
The neutral zones could be fairly empty places with very scarce populations , like deserts , cold steppes etc... Those would result in a difficult to settle for tribes but also civilized powers ... Also could feature random nomadic movements of nomad tribes .... So in a xxx region of the steppes , cold be controlled a large part by a nomad culture , with no main city but a moving city that can be placed in any subregional area that can hold it inside the boundaries of the macro region.... This wold simlate also the control of a nomadic tribe over a large area.... While a settled type of civ could instead convert to an agricultural base the large region a piece at time , subtracting so terrritory of living for the nomadic culture.... If the nomadicmculture remains without possibility to migrate inside its own territory is forged either to mass migrate somewherelse... Or convert to a different culture type...