To complement Brian's answer, you could use as a model the other, first Roman Empire, where emperors adopted their heirs, sometimes to great success. For example, Nerva adopted Trajan, who adopted Hadrian, who adopted Antoninus Pius.

You could have the ruler ask for hostages from the local tribes, raise them in the court, and choose from them the best successor.

To complement Brian's answer, you could use as a model the other, first Roman Empire, where emperors adopted their heirs, sometimes to great success. For example, Julius adopted Augustus, Nerva adopted Trajan, who adopted Hadrian, who adopted Antoninus Pius.

You could have the ruler ask for hostages from the local tribes, raise them in the court, and choose from them the best successor.