All the discussion of late regarding custom village crests, pmks, and the like have gotten me to thinking about thrones. The last episode of Game of Thrones was also a significant factor; though, I doubt one could escape the copyright infringement there (granted a good bit of legal scandal could be good for pr).
Atm we only have basic wooden chairs, no large obsidian monoliths, grand arrangement of bones, intricately-carved wood, or some other fantastical resting place for the royal posterior.
In addition besides having one generic throne model, or even one for each default kingdom, each pmk could have their own designs.
EDIT: Not sure what else could be custom for each kingdom, perhaps some sort of royal fanfare or trumpet call for when a king player enters local.