The Mario X Rabbids crossover RPG is something that has been rumored since before we actually even knew much about the Nintendo Switch- but it sounds like it is real, very much in development at Ubisoft, and due out later this year. According to Kotaku, they have been given assets that prove the existence of this game, called Mario + Rabbids: Kingdom Battle, which stars Mario, Luigi, Yoshi, and Toad in the playable roles (as well as four Rabbids dressed up as them…).
The game is being built on the same engine that Ubisoft uses for The Division, will have turn based battles as well as co-op, and will apparently be a rather funny game. As of right now, it is slated for a Fall launch, with what is expected to be an announcement this E3.
The very premise of this sounds so bizarre that I want to see it happen.