According to multiple job listings on Bandai Namco's Japanese website (via ResetEra), it's believed to be a remaster/remake of a '3D action game' by Nintendo. Bandai Namco is currently seeking planners to get the project up and running as well as visual artists to perform "HD remastering" of 3D backgrounds.
Here's a rough Google translation of one of these job descriptions:
"As a visual artist for a 3D action game project on a Nintendo contract, You will be required to perform HD remastering of the 3D background."
Bandai Namco has helped out Nintendo on all sorts of games in the past. One of the highlights of the Switch generation so far has been Super Smash Bros. Ultimate - featuring characters like Pac-Man and Kazuya.
What 3D action Nintendo game do you think Bandai Namco could be reviving? Leave a comment below.