With Mickey's Railroad coming to Disneyland's Toon Town, it seems likely the idea has been considered for TDL as well. I hopped on to satellite imagery to see how feasible it was...
Some numbers:
The show building for the WDW version is 60,000 sq ft, with the dimensions 300'x200'.
The show building for the Disneyland version is larger at 75,000 sq ft, with the dimensions 500'x150'.
My number for the Disneyland version is an estimate since satellite imagery is old, but I'm familiar enough with the site to estimate it. The extra square footage is likely accounted for either queue space or office space.
The limiting dimension is the width, so I made a rectangle with the shorter width, 150', and the same dimensions as the smaller WDW version, 60,000 sq ft. The length of this show building would be 400'. This rectangle is represented by the shorter rectangle within the blue space. The extra blue on either side is the extra length of the Disneyland version.
Obviously, the difficulty is squeezing in the show building between the boundary of the new expansion to TDS. I used the site plan at the beginning of this page to determine where the expansion's boundaries are. My reference was the building with the square "hole" in its bottom right-ish corner--the one cut off from my screenshot in the bottom right corner of the page. I extended the red line from said building to represent the Fantasy Springs limit. I gave enough distance between the two sites to allow for a backstage road, erring on the safe side. It's feasible to push the show building closer, however. The perimeter road around Disneyland, CA that parallels Harbor Blvd narrows down to one lane. It's a small inconvenience, but there's a precedent.
It seems like it is barely possible, assuming no optimizing of the floorplan to allow for a smaller building. With its placement, I sort of avoided any structure in Toon Town. Minnie's House is most in the way, but she does have a new studio as part of the Fantasyland expansion. So maybe it can be sacrificed. The easiest way to make room for the queue would be to utilize the Mickey meet and greet. But with some creativity it could be spared.
So, how realistic do we think MMRR is?