Solutions jump right out at you when you look at an overhead map. The area in purple (below) could easily be used to make a massive Villains Land, while still keeping a functional Rivers of America so as to not disrupt themeing for Liberty Square and Frontierland. And the Villains Land could now semi-incorporate the Haunted Mansion. This area is massive - matching the size of current Fantasyland. Even if you kept the entirety of the Rivers of America, the footprint is still big enough to match the size of the other lands in the park.
The Cars attraction could be placed in the area of the Tommorowland Speedway (in orange on the map) and, if you were willing to spend the money, you have plenty of room to build one hell of an attraction.
Of course, this took 45 seconds for me to create using Google Maps and Adobe Illustrator, so it's understandable that modern day WDI couldn't figure it out.
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