Yes, there is other means to become king. The most difficult would be to start from scratch a kingdom, being neutral and conquering a kingdom castle, and declare war on him, even if not a vassal.
Another method to remember is to convince the king to retire. I remember a save in the native (day 400 +) that I had an option in the dialogue with the king who tried to convince him to choose a successor. I owned something like 1100 renown
sry for my bad English,
Oleker