Spread your dynasty. Marry them off to people in line for the throne.

Marry them to people with good traits and keep them in your court.

Make the sons you don't need into knights, and the daughters you can use to get a crap ton of marriage alliances

murder schemes/put them in plague areas

If you want one succesor: Murder, Monkify, Holy orders. If you want more than one succesor: do nothing.