I wrote a long post about this before Imperator was released that even got replies from the devs. My suggestion was that powerful rulers of one cultural group can proclaim themselves hegemon. I expanded that if a culture group felt pressure from another culture group they might select the largest powers around them regardless of culture to be their hegemon creating a rift between those who want to unite under their own cultural hegemon and fight back or those who will accept the foreign hegemon. Maybe in places like spain you'll see these tribes split between roman and carthegenian supporters. Where as in gaul the split is between those supporting rome coming in and those against it.Sort of like the coalition system in CK2? The "Gaulic Coalition against Roman Aggression" if the perceived threat levels are at a certain point. A baseline of 50 to 100 aggression would cause smaller tribes to flock to one another. Though under this circumstance I'd say if Rome does win against this coalition they should be granted special AE and Warscore reductions for winning. This could be ultra over the top.
I don't know exactly how i'd do it at the moment but some awareness by tribes of the local super powers and an alignment options would be nice.
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