Sketchy as ever, but there we go.
If the human kingdom is Skaia, the trolls have Aqualia, though only the palace itself is in any part submerged in the sea.
Now, on to my usual breakdown of ideas and AU headcanons!
Karkat as a Knight - way more tattered and scruffy than Dave is, as you can easily see. Karkat is not of nobility, nor even from anything approaching a wealthy background. He taught himself to fight, struggling from the ground up. Knighthood wasn't even an ambition of his; he just happened to save the life of the right person and the next thing he knows he's being honored with that title. Not that it changed much for him; he still acts like a renegade, charging about the countryside saving people and killing baddies willy-nilly in his usual bad-tempered way.
Aradia as a Maid - Not entirely sure about her role in this...at least, not yet. It's possible that she's a valued maid in the palace, a good friend of the Empress to be, and possibly in a relationship with the court mage. Whatever the case, I've got to say that I love how her dress came out.
Tavros as a Page - poor little guy aspires to be a knight someday, but first he's got to get over his weaknesses - physical and emotional - and that's going to take a while. Still, he holds out hope for that wonderful future, where he's the one charging with the lance rather than handing them up to the knights.
Sollux as a Mage - court mage of Aqualia and a highly powerful psiionic, Sollux is nevertheless a pessimist, constantly fortelling doom and destruction. None of his predictions have quite come to pass, so they aren't really taken as seriously as he feels they should. Nevertheless, he's very good at his job, and while he's obviously close to Aradia rumor also has it that he and the Empress to be are a bit more friendly than formality demands.