The Mass Effect 3 Extended Cut packed a few minor ret-con tweaks, but mostly provided additional scenes to clear up the ending of the game. BioWare seems to be giving tacit admission that it should be considered the "true" ending, by deciding to include it on the Wii U version from the start.
"The extended ending is basically going to be part of the game instead," said producer Michael Gamble. "You won't have to download it."
We're still fairly in the dark about Wii U's storage capabilities. Gamble's comments to Siliconera may indicate the company is looking to ensure everyone has access to the extended ending, regardless of how much storage Nintendo plans on including with the console. Given the DLC was a hefty 2GB, it would make sense to include that data on disc, if possible.