Give Empires Cultural Domination
A recent Dev Journal highlights the Empires as being about force and military might, with men/women who are born to destiny and greatness.
I'd like to suggest that the Empires (and not the Kingdoms) be the best at Cultural Domination. That is, the neighboring kingdoms around them, those that aren't conquered by them, are subject to ruthless cultural indoctrination and expansionist politics that overwhelms their border cities, with the weak kingdom and neutral factions crumbling under both the military might and the persuasive, controlling cultural might of the Fallen kingdom.
This flies in the face of games like Civilization, where "culture" is a function of a peaceful nation, seen as an alternative to war.
I find that's not really realistic to have nations like France annexing other cities because they "have the better wine, and oh, look at that pretty metal tower." It's much more realistic to have nations that believe in their superiority, like for instance China, taking over surrounding neighbors and annexing them even without military action. Witness Tibet, etc.
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That's not to say cultural or nation borders should be unaccessible to Kingdoms. Of course Kingdoms should get culture and benefits, but they should gain access to the neutral factions through diplomacy and other options. Rather than being about a Kingdom oppressing another tiny nation out of existence, they should have the option of peaceful unification. Taking empire cities by culture shouldn't be about "my Kingdom is just so much stronger, you should join me," it could be "viva la revolution!" with oppressed people revoluting.