Make monster summoning less common, but make group spawns more common. (Not necessarily meaning group spawns caused by monster summoning.) In particular, when a monster that is a lord or prince to its kind spawns, it should always spawn in some associated retinue along with it.

Add a random throne effect that polymorphs you into a level-appropriate royal monster.

Croesus is able to displace other monsters, because he’s the king. Perhaps all king-type monsters should be able to displace other monsters.

Dwarf and gnome kings will never pick up worthless glass or gray stones on the ground (they continue to pick up all valuable gems).

Thrones have an affiliation with a monster. Only the affiliated monster can sit on it and use it. It starts out as the king monster generated on the throne. If the monster dies, its killer becomes the new affiliated monster. This would need some flavor for why the king doesn’t just sit on it and use it until it vanishes; there are a few possible ways to handle that. One is that though the throne effects are good, the king is flavored as not wanting to lose the throne. Another is that throne rooms will start empty except for the king and maybe a couple guards, and the king does in fact use the throne’s powers against you (without risk of it vanishing since he is the original owner, or something).

Knights instakill by checkmating all “king” monsters that are a knight’s-move away and are surrounded or cannot move.

King monsters should spawn with an entourage of monsters; e.g. a gnome king should create several other gnomes around him upon generation, with preference given to gnome lords and possibly gnomish wizards. If monster pets are implemented, they should behave as the king’s pets, and the king may generate with a magic whistle.