You are absolutely right about the scale of Civ4, and I doubt Civ5 will have any less of a "scope". You realize how large the scale really is when playing on Earth maps. Regardless of size there are bound to be countries in the real world that would only be represented by one single tile... This pretty much defeats the point of the milefield or any other "traps". Like, you would booby-trap the entire country of Georgia?
I do, however, think that a defensive improvement representing a warzone like the trenches of WWI could be implemented, even on this scale. And I do believe that minefields in real life are to be considered as defensive measures. Maybe Forts could have an additional defensive bonus with the advent of land-mines? (Sure, as the front-lines get blurred and the war rages back and forth, your troops are bound to get trapped in one of your own minefields. But a minefield, in real life, maybe the size of an acre. One tile in Civ4 - or any Civ game - could be the size of an entire country or region/state!)
I do, however, think that a defensive improvement representing a warzone like the trenches of WWI could be implemented, even on this scale. And I do believe that minefields in real life are to be considered as defensive measures. Maybe Forts could have an additional defensive bonus with the advent of land-mines? (Sure, as the front-lines get blurred and the war rages back and forth, your troops are bound to get trapped in one of your own minefields. But a minefield, in real life, maybe the size of an acre. One tile in Civ4 - or any Civ game - could be the size of an entire country or region/state!)