Bandai Namco is currently developing several NX titles. Smash Bros is planned to be a launch title. I am not sure about the date - yet ;).— Dr. Serkan Toto (@serkantoto) January 28, 2016
Dr. Serkan Toto, the CEO of one of Japan's premier game consultancy firms, claims that Smash Bros. will be an NX launch title.
The Nintendo NX may launch with one of the game developer's biggest franchises: Super Smash Bros., according to a reputable consultant in the Japanese gaming industry. Dr. Serkan Toto is CEO of Kantan Games, and has accurately leaked other advancements at Nintendo, including their Smartphone Initiative. Toto claims that Namco Bandai is also developing several titles for the NX already.
He followed up his initial Tweet by stating that he isn't sure if the NX version of Super Smash Bros. will be a completely new numbered version, or if it will simply be a kind of enhanced port of the Wii U version. Given that the development cycle on the series is quite long, and a new title usually doesn't release until quite a while into a console's life-cycle, I would hedge by bets on the latter.
Toto claims that his source on this is "solid", and he would be very surprised if it turned out to be false. Here's hoping!