They already have or at least have gotten in bed with some third parties to bring some good stuff to the Wii. I thought they were growing with their audience as the wii came out and Twilight princess was a more mature Zelda and then... Skyward sword, not that it was a shit show solely because of its kiddy vibe. Nintendo but themselves in an awkward position with their content where a large part of their audience falls off after a certain age and are forever in a cycle to get the young ones in off the backs of those who stick around yet the neglect the core audience (been a bot better with the switch but are lapsing again) in favour of having the grannies and everyone else in the family too. They can never grow across platforms this wag and its why they fall into these successes then failures, rinse and repeat,constantly chasing those exercising granny dollars. Of course that's a bit simplified but its a good reason for them to throw some money at their core audience who'll buy anything Nintendo because they seem to have dropped to about 10 million or so. But alas,they can't try to uphold this facade of being gamings Disney if they make mature games. I say do it and it'll be a positive for everyone involved core gamers, angsty teens looking for gaming on the go, folk who fell off of Nintendo for the sexs drugs and rock and roll of Sony or the teen Yankee gamers who only believe one should game on Xbox because there is no Weebs or childern. But some suit at Nintendo will disagree while tugging one out to a granny on a wii balance board.
That's a lot of oversimplification. When I talk about Mature casual games, I mean mature titles not aimed at the core gamer, and is simpler and easier to enjoy for less veteran gamers. Hardcore Nintendo fans aren't a big enough audience to build an entire console around, as the Wii U will tell you. Thus, it's important for Nintendo to diversify their investments to make a wide variety of games spanning as many genres and types as possible. The Switch should be a console that has something for anybody to enjoy, not just one type of thing for one type of audience.