One of the most talked about Nintendo characters in 2018 that isn't even an official Nintendo character was Bowsette. The female gender-swapped version of Bowser became an internet sensation overnight, and led to a lot of people requesting her in upcoming Nintendo games. Well, despite the vocal requests, she won't be appearing in New Super Mario Bros. U Deluxe.
Over on the official New Super Mario Bros U. Deluxe web portal, there's a full roll out of information on the game, including some of the old items making a return, and some of the new items you'll be able to acquire throughout your journeys. This includes information about the Super Crown, which is the item used in the fan fiction to turn Bowser into Bowsette. The idea is that when Bowser puts on the crown, he turns into the vicious vixen, Bowsette.
As the description above explains, only Toadette can use the Super Crown item in New Super Mario Bros. U Deluxe. When it's equipped, it allows Toadette to transform into a super-powered Peachette, which is a species-swap for Toadette into Peach. The description makes it clear that not only is this item new, but it's also only available for Toadette. Nintendo also comically shot down any potential appearance of Bowsette by saying "Sorry Luigi---only Toadette can use this item!" which means that even if Bowser were playable in the game, he wouldn't be able to get his hands on the Super Crown and turn into the curvaceous Bowsette.
However, while a lot of news sites are stating that Nintendo has shot down the inclusion of Bowsette, technically you wouldn't have to play as Bowser for Bowsette to appear.
Walk with me down conspiracy lane for a second: If Nintendo really wanted to flip lids, they could technically have a cinematic in the game where Bowser briefly gets his hands on the Super Crown and transforms into Bowsette. This would keep in line with the description stating that only allowing Toadette can use the item in-game, but circumvent that restriction by showing the Super Crown being used during a cinematic by someone else.
Of course that's all just speculation and hypothesis. There's no reason to believe that Nintendo would actually ever use Bowsette in a game.
The company has been coy when it comes to addressing the fictional character due to her... lascivious nature and the equally deviant nature of the fanart that has popped up for the game. Nintendo tries to keep many of its first-party titles fairly family-friendly, with the exception of Bayonetta, which is an M-rated IP that the company ironically rescued from Sega.
But I don't think that Bowsette's buxom bodaciousness would fit in well with the rest of the Super Mario Bros., universe right now, but maybe at some point in the future we'll see the popular baddie make an appearance in a Nintendo product?
In the meantime you can look for New Super Mario Bros. U Deluxe to launch on January 11th, 2019 exclusively for the Nintendo Switch.