The real thing holding back DK’s sales potential in the 21st century has been lack of Kremlings. Bring them back and Nintendo can fill a banana horde with the money they will make.
But actually… while I started this comment as a joke, I do think there is some correlation to it. Rare-era DK on the SNES and N64 kind of marketed itself as an “cool / edgy, but unlike Sonic is available on Nintendo” alternative to the more universally welcoming Mario. They really milked cool 90s kid interests and vibes with the franchise. I.e.: The somehow simultaneously dark yet colorful graphics were a selling point, realistic jungle/adventure theme, soundtrack full of bangers (culminating in the DK Rap), Taco Bell and Lunchables cross promotions, Diddy’s jukebox and hip hat, Cranky‘s ornery dialogue, Candy‘s flirtatiousness, Funky Kong’s existence, and K. Rool being marginally more threatening than Bowser, DKR was more chaotic than MK64. The list goes on. Some of these things
are present in modern DK, but it’s a different vibe. The difference in player experience between DKC and SMW felt more stark than, say, NSMBU and DKCTF. To me, at least.
Where Mario hit the Disney kid demographic, DK hit the Nickelodeon kid one. And, with nothing but love to post-Rare DK, I do feel like a lot of that specific brand of personality was lost when the Rare breakup happened. I think this “edgy” theming I am referring to has been delivered really well in the Splatoon franchise, so Nintendo can pull it off without Rare. And while a brand new Switch DK is probably going to sell great regardless, a return to its “90s era personality“ (which includes Kremlings) would be a unique feel relative to Nintendo’s other Platformers.