Once upon a time, people just had to fit things into the space they had. Strange that you mention this as they fit Carsland into the tiny DLR, surrounded by public roadways that they had no option of moving. Photoshop overlays show that they can fit it into DHS with a little imagination removing some attractions that few will miss.
Theoretically, if the cadillac mountain range of Carsland was pushed into the southwest corner of DHS, then this would involve taking out the backlot tour, LMA, 1/4 to 1/3 of New York street, cutting into Toy Story Mania, and a bit of the land for Honey I Shrunk the Audience and going up right next to the Muppets, in Carsland's present configuration.
However, if New York Street was extended to become the main street of Radiator Springs, and if they sort of built in a "U" Carslands up around the backlot tour, they would technically have enough space, but, you'd only have one entrance to Carsland, AND half of Carsland would be behind backstage areas to the west of Pixar Place. They'd have to demolish the backstage areas so that you'd have two entrances, but then the backstage area would be cut-off from the outside of the park, to say nothing of the extra cost involved. And the remaining backstage buildings would be an eyesore as they'd be right up against Carsland. Trees and greenery could be used, though with buildings that close, it would be hard to hide.
Plus, Luigi's would be moved away from its current location to the narrow part of the "U", thus changing the layout of Radiator Springs somewhat. This is a problem, IMHO, b/c Radiator Springs has a well defined layout based on the films.
Could be done, but they don't have the room to build Radiator Springs like it is in DCA, IMHO, and they'd have to do a whole lot more work than in DCA as Carsland was essentially a parking lot before construction, plus they'd probably have to demolish a whole section of backstage area and build an underground access road for trucks.
Possible, but it would cost more than DCA's version, might even be a tad bit smaller, possibly wouldn't have the "correct" RS layout, and you'd lose a backstage building or two, plus LMA and the backlot tram.
Wouldn't be impossible, but would be much more difficult than what was done in DCA. DCA's Carsland destroyed only a parking lot, in DHS, were talking about destroying two attractions, who despite current popularity, can always be refreshed at a fraction of the cost of tearing them down and building a new land. Yes, they tore down Sunshine Plaza at DCA, but Catastrophe Canyon and LMA are a lot more expensive.
Plus, the company concluded that DHS's possible expansion would be west of World Drive, no re-development and infill like in Epcot where they've got unused land for pavillions.