Perhaps an ability could be added based on spy or stealth factors. A spy counts as X number of units when considering intercepting an Army during a "Move into Enemy Territory" phase.

And allow 2 Map Movement Armies to have a map range that includes enemy provinces instead of just familiar controlled terrains. Or perhaps a Command in the Commander's Window called "Intercept Army" that allows you to have a 1 Province 'Defense" force that can move forward to a province that is being attacked based on a formula from your Intellegence.

This would have to not be affected by magical spells that allow Sneak attacks (Call of the Wind, Wild).

I don't think it would be hard to implement; per say, but I don't know the coding involved. Or if there would need to be another phase added into the turn process. But it would add another dimension to the game. Though hopefully not allowing people to turtle.