Once a realm loses its capital, turn all of their nobles into heroes with increased mortality rates. When captured, have them released in a random region. Have the hero drafting option more reliable.
Then, let them all fight to the death, no switching. If their capital was taken by a surprise move, they might just be able to reclaim it (unlikely), but mostly, if they still have regions, they can continue a riskier (for them) and less damaging (for other realms) guerrilla warfare, so that invaders have incentives to conquer the whole realm.
Just an idea inspired from the previous ones.
