Excellent point

I know Nintendo doesn't like repeating themselves for sake of being fresh and innovative, but some opportunities like those games mentioned above could do great with a modern twist in terms of the new hardware and software controls.

I for one never got to give Brain Age a go - did you play that one, I'm curious if it played as nice as it looked. ?

Could we say that 1-2 Switch was an update on the adult casual genre?

I guess we could, but I don't think it ever really jived with grown-ups the way Wii Sports did. Nor does it really deal in adult/mature themes like what OP is describing.

And even Wii Sports had a surprising amount of depth hidden under the hood. More so that 1-2-Switch as least, which is still a really fun game, mind.

That's a billion dollar idea right there.

Switch reaches a much more diverse audience than the Wii U ever did. And with games like Xenoblade Chronicles 2 and third party titles like Doom selling well, Nintendo shouldn't hesitate to try something aimed more at casual adult gamers. 

I have always been in favor of Nintendo making games that reach a wider audience. There's nothing wrong with sometimes getting a little away from what you usually do.

It may end up being like Love Plus...and then the guy that married his DS/game character?

https://kotaku.com/5409877/the-one-about-the-guy-who-married-a-video-game

Not sure if that's the best route lol

Whatever works, works.

I totally know what you're saying, but your two examples are anything but casual.
Nintendo did make some strides in adult casual, with games like Brain Age and Wii Fit, and some of the other stuff on Wii and DS.
At least if you mean casual in the retention-over-time way that it's used in mobile games now. And adult as in not really appealing to children.

They have apparently found this particular kind of successes quite hard to replicate though. They still have casual stuff like their mobile games of course, even if it's not utilitarian in the same way. And something like FE Heroes isn't something I'd say really have much appeal to children. At least, the children I know play Pokémon Go instead.

Now, OP mentions stuff like Hotel Dusk. And that I could dig. I liked the Trace Memory games a lot. But the target demographic was apparently so narrow that Nintendo eventually stopped working with Cing. Visually pleasant puzzlers like Monument Valley is something I'd personally also categorize as adult casual, and those I like a lot. I'd like for more of the stylish puzzlers from iOS to make it over, or for Nintendo to introduce some of their own.

Hidden object games with murder mystery stories could also fall in that category. And I think there's a few already on Switch? At least there was on Wii U. These fall quite far outside of anything Nintendo's been making though, but it'd be great to see them shape up the genre.

Of course, if what people are REALLY talking about in here is mostly story based adventure games with simple puzzles and a lot of character interaction, which both Night in the Woods and Hotel Dusk essentially are, then that's a genre that directly goes against most of the mantras that Miyamoto champions. Not saying they won't do it, but it hasn't ever really happened in-house. Cing was the only dev in the genre they ever really published for, and they're gone now. :/

Miyamoto doesn't run Nintendo's in-house development anymore, Shinya Takahashi does. Takahashi came from SPD, which specialized in working with developers like Cing with these kinds of games. SPD and EAD merged in 2015 to form Nintendo EPD, so we could see more games like that come from Nintendo's in-house teams.

Takahashi tends to be more of a risk taker than Miyamoto, willing to green-light titles that have no mass market appeal, but do help diversify the library, whereas Miyamoto was always about appealing to the widest audience.