This is a pretty good thread. A lot of good ideas and suggestions. Plenty of heated debate. A little bit of flaming. ( At least no one was hurt ).
I guess its time for me to stop lurking and post my suggestion. Actually, its not mine. It's just a refinement of a previous post.
Here it is...
Ground units can only be reduced to X strength via air attacks.
Note: I'm leaving the strength number "X" open for debate.
Positives: Easy to understand. Should be easy to code. It would have desired effect of reducing ability to rely on air alone on attack.
Negatives: Possible gaminess. Ring coastline with X strength corps and stop invasion indefinitely ( Although you could finish off the unit with a naval bombardment ).