There are many different ways to integrate the theme into a game. If you're completely lost, maybe try to think of it this way: the ability of being untouchable/inaccessible/protected is to be given to a certain element of the game. Could be the player, an enemy, a treasure, a key,... The goal of the game could be to try and break these boundaries, or maybe to reinforce them. So you're either trying to access the untouchable, or you're the one protecting it. Again, the sky is the limit, let your creativity lead you, these are just basic interpretations of the theme that might get you going. Good luck and let me know if you need more assistance!
