Super Mario 3D World + Bowser's Fury has data for an unused Giga Mushroom, which resembles the Mega Mushroom but with a red-yellow color palette and an angry expression with a cross-popping vein, a common anime trope used for anger. It has seven animations, but each one contains no data. Touching it plays the "item into inventory" sound. If Mario touches it, he will become Mega Mario, and if Mega Mario touches it he will become Giga Cat Mario, suggesting it would have been used for Bowser's Fury.

Super Mario 3D World + Bowser's Fury has unused character select screen voice clips from Toadette ripped from Captain Toad: Treasure Tracker, suggesting she was going to be a playable character at some point.

Normally the cats in Bowser's Fury would move away if Mario gets close to them, but if approached while in his Cat form, they'll instead walk over and nuzzle against him.

An early version of the game's box art features Cat Luigi in a similar position to the original Super Mario 3D World box art, alongside a placeholder section for Bowser's Fury displaying the campaign's logo. The final box art removes Cat Luigi entirely to make room for artwork representing Bowser's Fury.