There is currently no place to put Carsland at Tokyo Disneyland. They are not going to pave over the Rivers of America, eliminating the Mark Twain, Tom Sawyer Island, and have the view from the Western River Railroad be Radiator Springs.
In order to build CarsLand, they'll have to use the parking lot area, where there is room for Radiator Springs Racers. However, the backside of the Cadillac range is just plain butt ugly and would look terrible to all the people in the official hotels. However, if they build the multi-story parking garage so it backs up onto the Cadillac range, that might solve the problem. This would, I assume, eliminate the dreary Grand Prix raceway whose time has long come and gone.
Yes, that does put Carsland in that weird area between Tomorrowland and Fantasyland, and not in Westernland. But it can't fit in Westernland, and if that's the deal-breaker then they'll build a Star Wars land instead. They can easily demolish the theater where the awful One Man's Dream has been playing since dinosaurs ruled the earth and extend the new Star Wars land all the way from Star Tours to the edge of Fantasyland, just before Pooh's Hunny Hunt, and extend out into the parking lot. That seems a lot more likely to me.