After the king reaches 100+ bones or so (or 50 or 150 or 200, whatever seems best), give them 10-20 seconds warning, then switch them to the black team.
Switch their weapon to a grenade launcher that has unlimited ammo to shoot grenades at the cost of half a bar of stamina
Give them an additional weapon that launches a grenade at the cost of a full bar of stamina by right clicking while standing still/drops a mine as the special.
Maybe make the king drop additional loot/coins.
This would make being the king more fun, make it something people aspire to do, and make people potentially work together to bring them down.
Customer support service by UserEcho