Sigismund III
Sigismund III of Poland was for a few years in real life, simultaneously king of Poland, Grand Duke of Lithuania and king of Sweden. It was an unstable relationship because Poland was Catholic and Sweden was protestant, and eventually Sigismund's protestant uncle (Charles IX) who had been his governor in Sweden squeezed him out. With a more loyal governor, and preferably no protestant/catholic split at all, Sigismund's rule could have lasted longer.
Also in real life, Sigismund intervened in Russia in the Time of Troubles, sending his son Wladislaw with an army to Moscow where he was elected Tsar by some of the boyars. If Sigismund III dies shortly thereafter, Wladislaw inherits all his father's kingdoms.
Good luck hanging on to them.