I have an idea for a new decision, called "Encourage Vassals to Expand De Jure Realm." It only works for Kings, but this would maybe be a like a "Request Claim on Ireland" from the pope. Only instead of you requesting permission, you grant permission to your vassals to attack enemy holdings of your future De Jure Empire. Not sure what the benefits would be, maybe it costs prestige to take, but makes declaring what would maybe be called "Liege Encouraged Conquest" cheaper in prestige and/or piety. Maybe it begins to form a friendship with the vassal that does it. Maybe both, maybe something else entirely.