Come to think of it, they had probably always planned this for a Wii U release. The framerate dips on 3DS version start to become suspect. They probably wanted to end the Year of Luigi with a bang.I don't have a 3ds so I would love this to be true. If this will come out between Mario 3d World and DKC it would be really perfect.
Edit: it makes sense to be still part of the Year of Luigi, so maybe December?