These are just off the top of my head, just the basic themes.
Pirates – Puzzle Pirates doesn’t count. I want to be a swashbuckler on a ship with my crewmembers. I want to sail on the sea, find a merchant ship and attack it. First with canons and later by swinging onboard for some combat. I want to be able to bury treasure and I want to be able to search for treasure. I want sailing to work like real sailing.
Robots – I think robots would be fun because they would be so flexible. You could justify replacing body parts, even the whole design. Upgrade your robot. Changing your skills would make total sense. Go into a shop and download new software. Combat should destroy parts of the robot and you’d have to go buy a new arm or whatever.