I suggest migration from small city to large city and can be observed by shacks and huge slum area.

I suggest this because it will give different strategy of game and also suggest migration treaty between to civs which will result in drastic increase of migration.

Border fencing can be done near international border and help in keeping out hostile armies and illegal migration.

Border fencing and trench will give us defense against attacks.

 I also suggest bunkers here we can keep units like artillery or machine gun to defend territory.