I would have loved this in the old system. But now, we don't have ducal seats anymore... Much more complicated. And I suspect that you'd see many linear duchies that all place their ducal seat on the border with the enemy.
Seems like it could be discouraged by having some code that does something similar to the 'distance from capital' mechanic but on the duchy level.
Or an acceptable tax rate penalty on duchy regions once a duchy grows beyond a certain number of regions.
