Chief caster is a formal position in that Erfworlders seem to take it seriously for rank and chain of command, but it differs from Chief Warlord in being definitional -just the highest level caster on a side- rather than appointed, and doesn't confer any bonus like Chief Warlord does. That indirectly supports the idea that Rulers are similar - the benefit of being a ruler is you're the booping ruler, and we haven't seen any hint of global bonuses because there aren't any global bonuses.
And to the original post, I don't think you can have an unusual unit become King. An unusual unit can become Overlord (such as Charlie for one, who is probably a caster if not an actual intelligent free-willed Arkentool), but Erfworlders themselves are very conscious of the difference, presumably by reason of experience. The only known way to get a Royal ruler is by succession or spinning off of a royal, heir warlord. I'm guessing a caster Overlord has no special bonus either, but still, being an Overlord and also say a Thinkamancer or Mathamancer or several others is probably its own reward.
Slately and Don aren't very impressive, but some number of other Royals must have come to the throne after leveling. Imagine pre-Decrypted Ansom or Ossomer as ruler, they'd be much more formidable personally than Slately, more comparable to Jillian even if they did play it safe and traditional by staying home thereafter. Even without a global bonus, there's still a benefit to being a royal side in that your royals start with better stats and level faster, making them harder to take out. And possibly a high level ruler could appoint himself Chief Warlord for the global bonus, though it sounds like traditional Erfworlders just wouldn't.