As shown on Treehouse Live yesterday, Luigi's Mansion 3 includes a level that takes place in a movie theater. In one of the hallways at the theater, you can see all sorts of movie posters. The thing is, those aren't posters for movies, but posters featuring art from Next Level Games' previous titles!
The posters include tributes to Super Mario Strikers, Mario Strikers Charged, Punch-Out!! Wii, and Luigi's Mansion: Dark Moon. There's also a wolf poster in the background, but we're not quite sure what that one is for.