I hope I'm wrong, but odds are pretty good this is not a brand new Smash, but just a port of the Wii U game, as they seem to be porting practically everything else.
I think it would make the most sense to just port the Wii U/3DS version, bring back characters that were taken out due to the restrictions of the 3DS (Ice Climbers/Pokemon Trainer) and update it with newer characters and stages. Smash 4 on Wii U is already beautiful and flows perfectly, why start from scratch? They can save that for the next Smash Bros. Besides, Sakurai needs more time to heal up.
I honestly won't mind if it's just a port - the Inklings hint at new content and the gameplay itself doesn't change all that much between iterations, anyway. But there are two thoughts I can't shake. 1: The Switch is a hybrid between console and handheld - and there were both a console and a handheld Smash Bros with slightly different content. Could this Smash therefore - like Hyrule Warriors - contain content (levels, trophies, maybe even the Smash Run/Tour modes) from both? That would fit the whole DX or Definitive Version approach that is so popular with Switch games. 2: There is no longer any need for content parity between systems. Could that mean they might reinclude content cut due to 3DS limitations - like the Ice Climbers? Or will the new content be made available for the WiiU and 3DS versions as well?
Sorry, just idle thoughts I wanted to vent. But even if none of it turns out to be true, I'm still hyped.
I was worried that it could be a port, but I don't think it is for four (circumstantial) reasons: 1) That's "Breath of the Wild" Link; 2) the background music during the shot of the Smash Bros logo is not from the Wii U or 3DS versions; 3) the credits don't mention Bandai Namco; and 4) that's a new logo. While they could have updated Link's model for a port, the new theme and logo tell me it's a new game. The Bandai Namco part could just be the fact that they didn't show any assets they worked on in the previous games, so at best it's an iffy piece of evidence.