OH, MY. THIS IS A GOOD QUESTION INDEED. 😲😲 hmm. well, it depends on how "official" we're talking about. i assume official is like, lily ritk or ones we dont have two cards about. in that case, definitely retro rui and yokai kasa! they're so cute together! 😊☺️😊 (and yokai chuka is very very breedable... people who agree, please high five me!)
otherwise, i think i actually quite like an au where they're a little older. its a bit basic, but i think there can be a lot played once each member of wxs begins to pursue their actual dreams. the yearning rui feels and the distance between rui and tsukasa... thats very bittersweet
other than this, obviously, just general yandere rui. otherwise, i like any form of a royal au. maybe one where rui is the prince and tsukasa is the protector instead of vice versa? 😮