If you don't mind OP, I'll tackle each part separately, as you raise some interesting points:
Don't get me wrong, I am a Nintendo fan, though I don't love all of their franchises, but if you think about it Nintendo has been making the same games since the 80s. By that I mean games that are steeped in traditional, arcade design.
I think that's a reasonable statement. Nintendo have arcade DNA in many ways, and simplicity has always been at the centre of that type of game design. I'd say the games evolve as much as any other genres with their origins in the eighties - variations on a theme - but I understand when others don't agree.
I'd like to see Nintendo challenge themselves & deliver a few new IPs that have compelling narratives & deep gameplay like in The Witcher 3 or Fallout 4. I think it could really help in bringing variety to their current franchises.
But that's already a crowded field. Nintendo have managed to remain relevant,
because they failed to move into what others consider a more mature area, not in spite of it. This has left their output looking relatively unique. To change would be to vacate an area that not only mirrors their design philosophies, but has also proved lucrative.
More & more I find myself unable to be grabbed by Nintendo games because most of them lack strong narratives.
I love strong narratives, in fact, I'd often rather play a poor game with a good story than vice versa. However, I simply look for that itch to be scratched elsewhere. In the same way I don't play The Walking Dead for fun moment-to-moment gameplay, I don't wish for Nintendo to weave in thought-provoking narratives.
With Nintendo games, it's pretty much what you see is what you get. Colourful mascot characters collecting bananas & jumping on Goombas just isn't doing it for me anymore. As I'm getting older this stuff gradually appeals to me less & less. I need something that does that little bit more & goes that little bit deeper.
I'm the opposite. I'm increasingly turning to Nintendo titles because of the reasons you listed. It seems everyone loves the Souls series - deep, rewarding, "mature" - but I see a slog and a maudlin, oppressive atmosphere. Fans always find the variety in a series, and those outside always see less. The consistent art style of, for example, the Mario franchise often lulls people into thinking those titles haven't evolved, but they have, and significantly. Depth comes in many forms, and it doesn't always lie behind character interaction or impenetrable lore.
So, what do you think? I think making more games that focus on having strong narratives & a darker or more grounded visual style could really help diversify their offerings. After all, it's said that they make games for everyone, right?
They make games that are universally accessible; so the games are for everyone, however, this isn't the same as thinking their total output was ever intended to be varied enough to please all tastes.
On a side note - and this certainly isn't aimed at you OP, but the topic as a general theme - I always think of this quote from C S Lewis when people start questioning Nintendo for not offering adult or mature titles, so I'll just leave this here:
Critics who treat 'adult' as a term of approval, instead of as a merely descriptive term, cannot be adult themselves. To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence. And in childhood and adolescence they are, in moderation, healthy symptoms. Young things ought to want to grow. But to carry on into middle life or even into early manhood this concern about being adult is a mark of really arrested development. When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifty I read them openly. When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up.