It seems like if the gameplay hook they've come up with is something that doesn't fit with an existing IP, then they make a new one, like Splatoon with its ink mechanic, ARMS with throwing long distance punches, Ring Fit with the Ringcon, Drag x Drive with the mouse, etc.

In DKB's case (you mean Bananza, right?) I feel like the destruction mechanic is a pretty good fit for Donkey Kong, and as it had been over a decade since his last game I'm not complaining.

Do they though? I mean just look at all the Mario spin-offs. Would it be better if they were new IPs instead of Mario?

It is an arguable point.

Diversification is always important. One day Mario and other characters will be in the public domain. Creating new IP gives you new avenues to exploit. There are more merchandising and spin-off opportunities.

I think you're right. And I wasn't complaining in the sense of like, I don't think it's a good idea. Obviously right now, they can't seem to do much wrong, so how can it be a bad idea? But for me personally, I'd like to see new IPs from them. Also, its kind of tiring seeing the arguments against them for using the same IPs.

I think that's why I still like them to this day (at least, their philosophy when approaching game design).

It seems you can feel it sometimes when you play one of their games - You get this feeling that it could have been anything, but they decided to slap an existing skin to it. But somehow they make it work, probably because the art direction and everything else is built around the gameplay mechanics, not the other way around. Hey, it's been working for the past 30 years, so they must be onto something.

It is this kind of thinking that actually led to Nintendo being where it is today. Yotoi was working on a novel little gadget (called the Ultrahand) during work hours in secret, got caught, and instead of getting fired, he was praised for the idea. Once the product hit store shelves, Nintendo went from a business on the verge of bankruptcy to raking in millions in surplus. Following this success would come many more of Yokoi inventions, including the D-pad, Game & Watch, NES, GameBoy, etc. Miyamoto has referred toward Yokoi as his mentor and for valid reason. His influence still guides the company to this day.

Disagree as merchandising and spin offs are one area an IP like Mario has excelled in it probably hits more opportunities than any other IP.

Don't get me wrong, I'd love to see new IPs from them as well; really liked Splatoon for instance. 

Hopefully we'll see more original IPs from them with the new possibilities Switch 2 opens up.

Because from after the N64 and after Wii for fad IPs, what new IPs have had staying power? Splatoon from 2015 on Wii U is just about the only thing after Animal Crossing on N64 in 2001 that has staying power. Most of their core IPs originated on NES, Super NES, and Game Boy.

Switch Sports has sold over 15m on Switch two gens after the Wii, I wouldn't call that a fad. Ring Fit has also sold 15m plus.

Pikmin and Xenoblade have also shown staying power.