Fantasy monarchy concept that some anime has probably already done: A classroom of heirs-to-the-throne.
A relatively stable empire manages to keep the hereditary rule from imploding by establishing a system where only a very small handful of people know the true identity of the next heir to the throne. The next ruler is hidden among a group of about 20 randomly selected orphans, all roughly the same age as the heir, all assigned to be raised by different notable families. Besides being a great honour, the duty of raising one of the potential heirs is a great foothold to power, because even if their Chosen Child isn't the actual heir, the other children get appointed to other powerful positions depending on their personal talents once they're grown.
Being raised by their appointed families from infancy, the Chosen Childen are all given the same royal education, in the same classroom and by the same teachers. The entire point and purpose of this is that none of them know which one of them is going to rule over them all one day. Being an empire of unfathomable power, this kind of childhood upbringing is the only time in the future ruler's life when they have peers and equals, and by necessity they learn to both be diplomatic, and always wary of potential threats - you can't exactly bully the weird kid of your class just for shits and giggles if there's a 1 in 20 chance that he'll one day have the power to have all of you beheaded on a whim.
