Also since Essos was added a Warden has an obligation to defend the realm from external enemies in their region. So say the Warden of the North fails to answer a calls to arms against a King Beyond the Wall they would lose prestige and opinion throughout the realm.
It's already in, wardens are honour bound to defend their regions.This would be a great feature. If the system was created, it could applied to each Lord Paramountcy. For example, if Dorne is invaded by Summer Islands, then the Warden of the South is obligated to help. If a slave raid is made on the Iron Isles by the Ghiscari, the Warden of the West must assist.
True, but it rarely happened in canon as well. The wardens were only ever formally called up in the Ninepenny Kings if I recall correctly.But, it is rare that external powers will attack the Iron Throne directly. So far, I can only recall embargo wars.