I think having a choice of room shape would be cool, like in happy home designer. I also wouldn?t say no to a bigger house, I love decorating so the bigger the house the better. I also think it would be super fun if they incorporated some things from happy home designer where villagers could make a design request and the player could remodel their home for them.
I would LOVE to have backyards like in HHD. I've been wanting something like that since WW.
As for the interior, I think the space we have now is definitely enough. I honestly wouldn't know what else to add to my current houses! But being able to have more freedom in choose room size/shape might be cool, again like in HHD.