Uhm... I see more and more people believing the leaker, but I'm still sceptical.
Correct me if I'm wrong, we know for sure he guessed Little Mac, Palutena, Mii and Pac-Man.
- Little Mac returned in the new Punch-Out for Wii.
- Palutena was teased to death. Sakurai also developed Kid Icarus Uprising. Furthermore, he said he was going to put more female characters in Smash 4.
- Mii is also playable in MKW, MK7, Mario Tennis, Mario Golf, basically every Nintendo character-based online game.
- Pac-Man is Namco's rep.
I don't count Greninja as an actual prediction -- every Smash introduced a Pokémon from the latest generation, and of course Smash 4 is no exception. He simply said "Pokémon from X/Y".
The hardly predictable characters are Villager, WFT, Shulk and Chorus Men. The first two were posted after their reveal. The latter remain to be seen.
And he completely failed to predict Rosalina.