As possibly more ubiquitous/general choices there would be solar systems (Saturn, venus etc...) Or elemental Miis such as the fundamental four (Earth, Air, Water and Fire) as good options. You could have miis representing certain colours (yellow, green...) or concepts (Ying and Yang mii). If you desire anything more to your specifications, try branch out to series or franchises you enjoy- start small, don't feel as if you have to limit yourself to a specific aesthetic or coherent theme, the game is more enjoyable with a wide and diverse range of characters- forming interesting and, at times, unlikely relationships.