Thank you very much for the detailed end in-depth answer, every response should be like this. I shall take this all on board.
Then be prepared for more. I've been manipulating this terrain for many years, and I've developed some definite ideas about how it should, and could work.
A long standing, personal bugaboo. A Roman Warrior in 3,000 BCE should not be able to walk to China!
Alexander the Great's men walked to India. Someone crossed the Bering Strait long before that. But I see your point. Let's say you could arrange it so that a unit that is not within its Civ's influence, and not connected by road, port, or other means, would lose say, a tenth of a hitpoint for each turn it remains separated, unless 'healed' by rest (say, 2 turns, unless 'trained' which would reduce it to 1) or food. I assume 'corruption' was originally meant to cover that..
That makes me think: perhaps irrigation, mines, roads and rails could be made to deteriorate (roads fade, fields
darken..) unless re-worked occasionally. That would make things more interesting.
Which brings me to a big, glaring, awful oversight that has always existed: Healing units. I don't know whey they were never included.
Surely if the mechanism exists for buildings to improve mood, heal units, increase trade, etc., might we allow units to do the same in the square (or range of squares) they occupy? Suddenly, food, priests, medics, food supplies, drugs, merchants, spells, booze, cops, celebrities, EMTs, politicians, and many more units would have a purpose and function beyond brute strength or terraforming. Interestingly, many units of this sort are already in the database, waiting around for something to do.