Nintendo and Bandai Namco have partnered on a number of games, most notably Super Smash Bros. Ultimate, but also New Pokemon Snap and others, and it seems a new collaboration is coming.
Bandai Namco recently posted listings looking for a planner for a 3D action game project, as well as a visual artist for this 3D action game. What is notable is that this project is a game contracted by Nintendo. These listings can be seen here and here
As mentioned earlier, Bandai Namco and Nintendo working on projects together is nothing new, but what is interesting is that another listing states this project to be a remaster/remake, and the company is looking for someone to work on the HD remastering of 3D backgrounds
There are a large number of games this could be, and it will be very interesting to see what this ends up being.