You're not listening. It was a concept that was scrapped in pre-production. Epic Mickey was dark in concept, but not as dark in art as the leaked artwork. That artwork took on a much more realistic and dreary tone. We still got stuff like the mechanised Donald and Goofy, but they were stylised as something a little more comical over the more realistically proportioned and spooky versions in the original art.
Disney were unhappy with the first round of concept art, as they felt the imagining of Spector's world was darker than necessary, and Spector agreed. They toned it back to something with a dark streak, but with a more vibrant colour palette and 'cartoon' look.
The game you're imagining didn't exist. It never existed. It was an idea, an idea scrapped because it wasn't the art direction they wanted to persue. This happens with every game. Pre-production art circulations, concepts are shared, and eventually a direction is chosen.
At best you'll get an Epic Mickey Wii2 that looks more like the original Game Informer cover, or like the concept art that was unlockable in the game. A cleaner presentation with far greater geometric and texture detail, with the added benefit of strong shaders, lighting and shadows to bring the worlds alive.
Expect it to look like the leaked concept art is being ignorant of why that direction was never persued in the first place, which again had nothing to do with the Wii's horsepower.
Colours, tones and proportions. For a better example, compare the animatronic characters from the original art to what we got. Less emphasis on gritty details and dreary tones, stronger focus on a vibrant colour palette and toon body proportions.