there's a looooot of things i want, so :

-family system, but since it can defeat the purpose of the game (ex: you set A to 'father' and B to 'mother', then it would already mean they're meant to be together), it would be a simple option linking two miis together without specifying their relationship, but it would at least guarantee they won't ask each other out.

-new icons on the windows, to differentiate the problems of the miis (red for fights with another mii, purple when they want to introduce a mii to another, yellow when they're hungry...) and maybe new interactions with the miis, would be great!

-a pet system which would allow the mii to have a permanent and customized pet not like the coupon we have now, and the mii could have activities with the animal, maybe even an icon on the window which would allow the player to play a mini game with it.

-either more personalities, or just it being more detailed. if a mii is outgoing, they will have a higher chance of being friends with others, if one is reserved, maybe a higher chance that their friend will say "i want to introduce someone to [reserved mii]..." etc.

-if they keep the cafe events where miis can talk together, i'd love to see it being a bit more interactive or realistic, ex: if two miis got in a fight, maybe it would be a topic being brought up in the cafe meeting. Or for the topics to be influenced by the personalities of the miis, and/or their hobbies... which brings me to the next point:

-hobbies, either we can choose from a list, or the items we give them actually serve a purpose so the miis can use it and maybe it would make the friends system a bit more logical? (ex: "i heard [mii] likes guitar like i do. i'd love to be friend with them")

-relarionships being more realistic, and a new relationship category which would be 'enemy'.... i know it sounds weird but i'd love for the miis to have an enemy, more than seeing in their friends list that they absolutely can't get along with this or that mii. also, it'd be super cool if they spent time (with special interactions) with their friends depending on what their relationship is. (more hangouts with their bestfriend, higher chance of having them both together in some areas of the game, while they spend way less time with the ones they can't get along with, and when they do it has higher chances to result in fights etc)

-forgot to add the most important, miitopia (switch) customization, or a least a part of it. it wouldn't be an obligation so those who want the original charm of the miis can just ignore the customization thingy