Breakthrough Hit: For Ubisoft Milan, which had primarily worked on low-profile handheld and "casual" games as well as support roles on Ubisoft's bigger titles until then. The success of Mario + Rabbids caused the number of job applications to increase 10 times as much and launched plans to expand the studio.
Content Leak: Internal documents for the game were leaked a few weeks before its formal unveiling at E3 2017, revealing the game's name, premise, and style, among other things.
Cut Song: The Phantom was actually supposed to have 3 acts to his song, each of which would play during phase transitions, but the second act was cut from the game. It can be listened to here.
Missing Trailer Scene: While Peach's actual introduction still fits her being "The Badass Princess", her Let's Get Dangerous! scene in the pre-release trailers isn't in the actual game.
No Dub for You: In regards to the scientist in the intro and the Phantom, this is the first Raving Rabbids production with full voiced dialogue to NOT be dubbed in the Rabbids' native French (for those not keeping track, the previous Rabbids productions with full voiced dialogue prior to this game were Rabbids Go Home and Rabbids Invasion).
Saved from Development Hell: A crossover between Mario and the Rabbids had been on the table at Ubisoft since the Wii era, but nothing came out of it until David Soliani stepped up with his proposal. Prior to that, all that existed was the basic idea that it would act as a subversive, self-aware take on Super Mario Bros. The only known piece of concept art dates back to 2010, depicting the Rabbids stealing Bowser and Mario chasing after them.
So My Kids Can Watch: A variation; because the game takes heavy inspiration from X-COM, the director of the franchise has said that despite not working on the game, he's looking forward to it as an "XCOM game" he can finally share with his daughters.
The Phantom's song was initially going to have rap version and metal versions for different stages of the battle, in addition to opera, but it was later decided that it didn't work that well.
Working Title: When Grant Kirkhope was brought in to work as the game's composer, it was named Rabbids Battle Kingdom to hide the significant inclusion of the Mario franchise.
Grant Kirkhope's work on this game indirectly landed him a gig to rearrange his Spiral Mountain theme from Banjo-Kazooie for Super Smash Bros. Ultimate. Masahiro Sakurai wanted Kirkhope on board but he didn't have Kirkhope's contact information, so he requested Davide Soliani to be his middleman in passing on his message.
and launched plans to expand the studio.