Recent recruitment information shows that Bandai Namco is currently recruiting employees for the work commissioned by Nintendo. Attentive netizens noticed that Bandai Namco was hiring some staff at its Tokyo studio. All job listings contain the words "3D Action Game", "Nintendo Commission", and one of them also contains the words "3D Background HD".
Unfortunately, so far, these are the only clues we currently have about Bandai Namco working for Nintendo, as the rest of the job listing doesn't mention any other possible clues.
Bandai Namco is actually not surprising that it is developing the Nintendo IP, as they have also participated in many Nintendo games before, including Pokémon, Mario Kart 8, ARMS, Nintendo Star Brawl, etc.
We could spend the whole day guessing what game Nintendo commissioned Bandai Namco to recreate, which looks like a replica of a 3D action game. Netizens speculate that this may be related to "StarfireFox", "Galactic Warrior Prime", "Myth of New Light" or even "The Legend of Zelda: Twilight Princess" and so on - but it is still uncertain.