Kingdom personalization ideas.
In addition to the excellent ideas already presented, here are a few more. I do not know if they would be reasonable from a programming perspective or not.
1. Soldier customization - mild differences in (same grade) gear, perhaps colors, amount of wear, and mildly different textures of armor, weapons and accessories. Eye patches, missing ears, limps and tendencies to spit on the ground.
2. Population customization - hair/eye/skin/coloration, height, build. Your people may look like tall blond Vikings, or short, lithe, tanned Romans...but you would have recogizable population traits. Once you interact a lot with another kingdom you may start to see children with mixed traits as your citizens inter-marry.
3. Architectural cutomization - stone/wood building materials type, design type (light and open with porches, huts, tipis, etc
4. Town lay-out customization - Your cities may be made of fishermen and follow the shore of the stream or ocean, long and narrow. Or your population might be farmers who tend to spread out with room between houses, or perhaps you have tight and crowded cities so it is easier to enclose them in a protected wall. You may lay your streets out in organized grids or you may have them twist as the follow ancient cowpaths.
5. Settlement/Agricultural customization - you might have land ownership based on a grid system like in the US, or in narrow strips extending back from rivers (like in Quebec), or in scattered and intermixed inherited holdings (like in Normandy and South England) or another form.
Other 'customizations' might follow your initial civilization and channeller choices. For ecample, if you choose scouting/ranger type strengths, your citizens may tend to wander further from the cities, establish more hunting camps, and give you better information about what is happening in remote parts of your kingdom as well as soldiers dressed in forest garb. If you are an engineer, you may not only get buildings built faster but they may also look better. If your leader is lazy then the people will take a cue from him and you will see piles of offal behind houses, more rats, and more soldiers with obvious warts and sores on their faces. If you are a beast master (beast tamer) then your citizens would have more pets in town, dogs and cats and monkies and parrots and the occasional raccoon. Hmmm, perhaps you could train units of skunks to defend your cities or make the enemy capitol uninhabitable.