Carsland in DHS would be great, if they can get RSR running consistently, which it was not on my 4 day visit to Disneyland in November. MJJ is a good kiddie-thrill ride, and DHS needs that (and more). Luigi's is a failure of a ride concept, so not building it seems like a no-brainer, excuse of water table or not.
I hope, though, that a smaller Carsland at DHS is done as part of an expansion of Pixar Place to include small lands/areas of a number of movies. This doesn't have to be done extravagantly or particularly expensively to be succesful. Toy Story Playland at the Studio park in Paris is a wonderfully themed land that is missing an actual good ride. At DHS, the good ride is already there, and they could add a little Toy Story Playland area with two of the Paris rides, thus making a little Toy Story Land. Then bring in the Monster's Inc. coaster and/or Ride n go Seek (or better yet, both rides in the same building and sets), along with a couple of snack areas. And then a smaller Carsland with MJJ, RSR, Flo's Cafe, and about 2/3 of the street that is in DHS. Now you've got four Pixar movies with substantial, but not overwhelming, representation in Pixar Place. Do it over a 5-6 year period and watch the crowds grow.