In war you can “cut an attack of at the pass” so to say. This is another demention of defense that would be very cool. If we had the ability to hit an attacking march on a clan mate, it could effectively end the march with a victory or reduce the march strength in the case of a loss of the attack. How does this sound?
It would be quite difficult to find an enemy march, much less attack that march before it hits its target. The defender already has a large advantage. Just throw your army on their walls and you should win. Only time you wouldn't is if the attacker is much stronger than the defender. In that case however, the attacker's march speed should also be relatively high so my previous comment applies. Good idea, though I do not believe it will be viable.