He's all but confirmed that he is doing something in January. Smash? Kid Icarus? New IP? Maybe even developing something for Sony and Microsoft? I would love a new Kid Icarus myself.

Whats this about confirming something in January?

I hope he's working on a 3D Kirby, 2.5D Kirby, or Kid Icarus Uprising 2 or something that effect. Smash will most likely be a port. I mean, if you're going to make something to show off NX, the IP has to do something enticing. Like Breath of the Wild has shown other Nintendo franchises, it's time to evolve, even if just a little bit. Maybe the 3D Kirby minigame in Robobot was a test for what a 3D Kirby could control like. It's time for Kirby to go full 3D.

He's in that "I can do whatever the hell I want" level with Nintendo so whatever he's up to, it's going to be 100% because he wants to do it. I'm guessing a new IP or a revival of an old one since he doesn't like doing sequels and he's done with Smash.

Maybe he's working on a new IP?

Of course, once they fully unveil the NX, they'll probably ask Sakurai about Smash 5.

Smash 4 Remastered

i would love to believe that he is working on a port of SSB4 to the NX, possibly adding new characters, hopefully adding Ice Climbers back in. i wouldnt say a new Kirby because well, we just got one.

He says he's done with Smash but I wouldn't be surprised if he convinced himself to go back to it like Kojima did every time Konami decided to make another MGS after he said he was done with the series.  I'd prefer if he moved on from it though because he has a rare gift of coming up with fantastic play styles that click with mass audiences and it would be a shame if he spent the majority of his career tethered to one series when he could be building the foundations for multiple new/revived IPs for Nintendo.

Is Sora a 3rd party? I feel like maybe it's not, but I'm not sure if there's actually much information on that. Though, even if they are a 3rd party, it's hard to see Sakurai working on a Sony or Microsoft game. Maybe not impossible, but improbable.

Probably something different from Smash for now because he's done for now. Whether he's done for good in Smash is something that will be up in the air for a long while. We just got a new Kirby this year so that's out of the question. Hopefully he'll change his mind and do a sequel to Kid Icarus Uprising due to its success both commercially and critically. I also wouldn't mind him, along with Aonuma, doing a new IP for Nintendo. Imagine these two doing new IPs... That would be great for Nintendo, even if they become half as successful as Splatoon.