A game like Killer Instinct or Perfect Dark would be great. What I miss most on current Nintendo systems is games for grown up players. On SNES and N64 there were plenty of these games to enjoy. Nintendo just became childish and uncool with the Gamecube.

Yeah i guess, thats a given though. 

Actually N-Space was the primary developer of that game.

I think the OP answers his own question, Nintendo should just have 2nd party studios to make those types of games and continue to create the content they are known for. Maybe they need to create and buy more established studios to be more balanced at bringing these games to market on their systems, but Nintendo themselves shouldn't really focus on M rated for the sake of making M rated titles.

Nintendo should have more sports games: Strikers, Sluggers, Tennis, Golf, Wave Race, 1080º, ExciteBike, F-Zero etc with all that modern sports games requires of on-line, championships etc.
Nintendo should build their own T-rated sandbox game. Something like an extended Lego Underground.
Nintendo also need simulator games - something like Cities: Skylines.
Nintendo could be good at T-rated RTS. Something like Starcraft and Total War.
Nintendo could build their own T-rated FPS game with inspiration from Timesplitters.
Nintendo could popularize bullet-hell games like Sin & Punishment.
Nintendo should transform Mario Galaxy to something inspired by No Man's Sky, with different themed planets.
Nintendo most of all need is a toys-to-life game where all mature gamers can play with their amiibos!

yes

Nintendo is a video game company. there's no excuse for them to ONLY cater to younger or care-free adult audiences. they could make some pretty sick M-rated games, just imagine a darker version of a Metroid of Zelda or something. or something akin to Eternal Darkness on the Gamecube

they've proven a few times in the past that they are very capable of making M rated games they just need to actually make some more!

seriously, they're not going to lose their parent/kid audience by having a few more adult themed games. As long as they're not overly gross or disturbing in content they'll be fine

I think my #1 pick from Nintendo would be for them to work on some horror IP. that is probably their biggest empty category and they've had some successes in the past in the genre but not in a while. 

Nintendo has tried numerous times to sell Mature games.  Die Hard, Eternal Darkness, Giest, hell - they even tried to market a CoD.  Bottom line is that Nintendo doesn't know this market, and refuses to learn.  Their online policies reflect that.

Right but it's a great IP.  No need to create a new one before going back to a legend.

A dream come true!!, sadly that's never gonna happen tho.

Nintendo needs to diversify their offerings maybe not everyone want a M rated game but the more varied range of games the better to a console catalog.

Maybe they can team up with Platinum for a new M rated ip, besides a new Bayonetta, and they can commision an M rated game ip to a western partner studio.

They should either buy or form a Western studio for the purpose of making "mature" first party games. And give them actual creative freedom instead of throttling them with conservative restrictions.

It's a massive slice of the market that they are almost completely missing out on.