The idea for this one honestly came from a simple thought experiment: "What would Wario's Yoshi look like."
I came up with the idea of a rat companion of sorts that could play in co-op or in a lost vikings sort of style, but I realised pretty quickly that I would lose a lot of the exploration feel of Wario Land by having to manage different characters. Then it hit me. Wario could become the rat.
Using a bell he recieves in the beginning of the game from a cursed castle, Wario can transform for a limited time into a rat that can eat money. Eating money serves as your way of collecting it, and will mean you have to find creative ways to get at it in rat form. The rat form can deform like Wario did in Wario Land 3, having you crush, feed and squeeze Wario to get where you want to go. But the rat form can't fight. Normal Wario has all his power moves and can deal with enemies and bosses, but can also take damage. The later game will have you switching between the two rapidly on the fly to get through the harder areas,
With the previous games being themed after cultures, and the rat transformation having warewolf like aspects, I picked a castle location. Since one of the only obvious transportation the Wario Land games haven't started with was a train, it seemed to fit.
I could probably have done some pretty intense backgrounds for this one, but I wanted to make it feel like a Wario Land game on first glance. A fun experiment at the very least.