This was always an issue in Rome. People were afraid their heirs would turn out to secretly be wannabe dictators, and you couldn’t do anything about it since you were dead. How would you go about making sure your heir was as close to your vision as possible, both outwardly and in their desires should they be emperor?
Meritocracy is the only way of success. The only question is by what or who’s measuring stick. I would look amongst the judiciary for those with a good record, a history of wealth without exploitation and a long history of selfless idealist altruism put into practice for the benefit of the majority and in opposition to corrupting pressures.
With so much power, coalition building is less important. Selfless altruism is the best case scenario in my opinion.