Avatar would definitely lose its charm if it were an adult show..
Can't say that
Slice of life and drama mixture does not have charm - most The Last Airbender episodes were a mix of Supernatural, Adventure, Slice of Life, drama and action. However in The Legend of Korra there is too little
Slice of Life/Adventure genre and too much action.
If "The Legend of ..." becomes an adult(moves towards explicit content genres) show, it will have to deal with more adult content and contexts. Some plausible options to do a 3 book series for the character of adult avatar(in the context of his universe) are a world war, suppression, theocracy, apocalypse... As the world of the Avatar is developing(so far cars, electricity, urbanization, industrialization, weaponry, planes), more genres can be taken into consideration.
Personally, I would like to see "The Legend of ..." in horror, extreme survival or psychological horror genres.