If you've played many different types of games, you may prefer to consider specific mechanics before further theming your game. When taking this approach, think about the games you like and ask yourself what you really enjoy playing those games. Is there a growth mechanism in the game? Are you satisfied when the game allows you to interrupt your opponent's moves and counter them? Do you prefer strategy games to cooperative games? Take your own game up a notch with these specific elements - or try combining them in new and fun ways. Maybe you hate the way a game prevents you from playing a certain way. Eliminate some pesky rules and see what you end up with! New games - like almost any creative expression - are usually remixes or reimaginings of old games. This is not a bad thing!
Obviously, these technologies do not exist in a vacuum. Maybe halfway through your process, you'll discover a new theme that makes you reconsider some of the game's mechanics that don't make sense in narratives. This is part of the whole process and brings us to the next part.