The king isn’t really that important of a class. He isn’t hard to kill, he only gets 2 votes in one trial per day, etc.
The following buffs would give king the power he needed to actually feel like a menace.
Butler poison
Unless the neutral king has voted guilty on a blue dragon member that was subsequently executed (or decided the fate of a cult/unseen member who died later) butler should die from guilt after poisoning the neutral king.
Royal finger
It becomes a passive
If whoever the king is accusing is only one vote away from being on trial, the player will be put on trial. This only works if the king was the first accuser (similar to how it is determined with hunter’s retribution). This will only activate once per day.
this is a sub of royal finger. King would have a 2 use night ability that made all his/her accusations count as 2 the next day.
Death immunity
death immunity nights 1-3. This would make it harder to be killed later on in the game, as you wouldn’t have wasted any guards.
The king should be a class that has more vote power and is harder to assassinate. Keep in mind that unseen can still poison him if the physician is dead, so it not impossible.