All rumors suggest a Smash port is in the works. One rumor, by Dr. Serkan Toto, said it was even considered as a "Plan B" for launch, in the event that Zelda was delayed again. This would again suggest it's a port.
And it makes sense. Smash is one of Nintendo's biggest sales drivers. Not only for software, but also for amiibo. Smash 3DS and Smash WiiU had an enormous amount of work put into them. And there is like over $100 worth of DLC, between the fighters, stages and Mii Fighter outfits. However, the WiiU's low install base no doubt capped that game's potential. The Switch could remedy that.
The best part is, Smash for Switch could compile the content of both the WiiU and 3DS versions, along with the DLC, creating the definitive edition of the game.
Thematically, it's quite appropriate — the Switch is both a console AND a handheld, so it makes sense to feature both the WiiU and the 3DS content. Plus the DLC.
A new installment, not part of the original WiiU-3DS proposal, also means Sakurai is free to break roster parity and add extra characters. He managed to make seven new characters for DLC with just a skeleton crew. Now he'd have a full staff again, and more time.
He can say that since this is a new game beyond the original promise, it can have new characters, and that since the Switch has more power, complex characters like the Ice Climbers (with Nana's AI) and the Inklings (with their ink splatter effect) are now possible.
You may also recall there was a job listing posted briefly by Bandai-Namco for a Smash 6 (the 3DS being internally codenamed Smash 4, and the WiiU being internally codenamed Smash 5). You may also recall Sakurai saying, near the end of the DLC, how he was planning to take a very short break, but he already had his next project lined up.
A definitive edition of Smash 4/5 — call it "Ultimate Smash Bros. for Nintendo Switch" — would also be the perfect sales driver for subscriptions to Nintendo's paid online service.
It's not confirmed, but there's certainly a lot of smoke. Here's hoping we see something at E3!