Typically kings only appear in fantasy, and perhaps alternate history, out of the speculative genres. I think that it would be interesting to see more kings (and queens) in other genres. Kings (and queens) in space? Sounds awesome.
When using a monarchy, pay close attention to the impact of such a government on a country. The social system in the country does not have to be feudal, though it often is. Taking a king and planting him at the top of a Congress could make for a very interesting system (like England had historically, sort of).