Actually, I had heard this as well. Unfortunately, it was a friend of a friend story. What my friend told me was that one of his friends got to go on a tour of SquareEnix's place, and Nintendo, and passing through Nintendo, he was told that Isaac as well as a few other characters mentioned by name (Shadow, replacing toonlink with dark link, etc.) were going to be in the next SSB. However, it WAS a friend of a friend story, so you know.