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.