A question and a suggestion

[The Finn]

I don't think turtling would be a big problem - in most 4x games - against humans at least - turtling will eventually mean your destruction.. you may delay it a bit, but a larger production base tends to win out in the end.

Only allowing the interceptor to intercept one army if the attacker divides his forces and sends them different ways still allows for the 'raiding the rear' scenario.. but such an attack will cost the attacker - and it does reduce how potent those rear raids can be by reducing the guess work in determining where they'll go.

Add in the ability for the attacker to choes a "target force" - one that makes itself known.. one that's obvious about where it's going while a second force is less obvious about it's intentions and you counter the reduction in the 'raiding the rear' effect with the possibility of drawing the intercept force to where you want them.

Again, it might be difficult to make the AI make good use of these tactics, but I think it would be great fun in a human vs. human game.

Not everything is production based

I can see some abuses if you could protect your territories with Intercept a target force commands. But a general command structure of "Intercept" would allow you to cover more of a provincial defense as well as making units like Knights, Horsemen and other types more in tune with thier cost if they can evade Intercepts to strategically strike past an Intercepting type of army.

I don't think worrying about how the AI will deal with it, is as a major issue as seeing if it would be a viable command type.

As it is; the AI is lacking in a few areas that can use improvement; and I would expect an advanced feature like this would be rudimentary at first, or allow the AI to cheezy and know when you are intercepting and exploit it

[Delphinium]

One idea here may be to allow armies with strategic movement of 2 to "patrol" in an area and intercept any enemy armies that move into adjacent territories.

This would be a double edged sword as if a big army appears...

The other side of the coin is that if we had only a few more closed borders between territories on maps, then we would create strategic choke points.
The sheer "openness" of the geography on most maps appears to be cause of this problem.