Everyone from Smash 4 returns as is, bar some minor balancing tweaks---fixing Olimar now that he doesn't have the 3DS limitation for one. Though they could bring back Zelda and Sheik as one character, I think staying as split-characters works well enough.
Returning Veterans
* Ice Climbers: The most obvious addition to the Switch Port since they were veteran characters cut due to the switch to the 3DS, Popo and Nana would make their grand return. Realistically I think we might ONLY see them back in the initial launch of Switch Smash (with one or two DLC characters added later), with the focus being on adding all the 3DS stages as the main reason to buy it, but that wouldn't be fun for a speculation contest to just list them.
* Wolf: The other veteran who seemed oddly absent, especially with a new Star Fox on the horizon, even though Star Fox Zero kind of fizzled out I think there's still a demand for him to return. Did Sakurai ever state why he didn't return?
Newcomers
* Chorus Kids: It seems highly like they were planned for Smash 4 but were cut for reasons unknown, and Smash Switch seems like a good chance to go through with their inclusion especially as I recall Rhythm Heaven Remix doing pretty well and bringing attention to the I.P. worldwide. It's assumed they may have been cut because they wouldn't work on the 3DS version, which isn't an issue now.
* Inklings. Another Male/Female character like Villager and Wii Fit Trainer, this is THE newcomer as far as I'm concerned for the next Smash and the only new I.P. released by Nintendo which I think is worthy enough for Smash without feeling like a "flash in the pan" addition. They're already iconic, have a ton of potential moves and an interesting style, and an Amiibo. Would they make new Smash-version Amiibo for them I wonder?
*"Roam" (Zelda Botw Character): Who the fuck is Roam you ask? That's what I'm calling the birdman seen in the Breath of the Wild trailer as he resembles a character seen in the Ishinomori Zelda manga, who was also a birdman archer. Anyway I thought with BOTW being such a big game for Nintendo, why not have a main character from it pop up in Smash Switch for cross-promotional synergy? Zelda didn't get a single newcomer (Zelda Sheik split excluded) in Smash 4, and this guy seems like he might be a break-out character like Ghirahim was who popped up in an early Skyward Sword trailer. Interestingly there's another bird archer who I think would fit Smash Switch, Decidueye, but Pokemon has plenty of representatives so I thought I'd take a chance with someone unexpected.
* Dixie Kong: One of Nintendo's greatest female characters in my opinion, her absence in Smash 4 felt a bit off with Tropical Freeze having just come out and I think she deserves inclusion.
* Itsuki Aoi + Chrom: Yeah, give us the MC of Tokyo Mirage complete with his Chrom Mirage as a cool "Stand User"-type character---another Fire Emblem representative, without technically being from Fire Emblem, and let Chrom finally join the fight but with a stylistic difference from Marth. Don't dub either and keep them talking Japanese though, so Sugita can play a Smash Brother finally.
* Rayman: The only 3rd Party addition, since Nintendo and Ubisoft are apparently working together for a Mario + Rabbids Switch game.
So, yeah, that's eight newcomers in all. Probably not gonna happen but, hey, I go all out with these.
Three new stages for tie-breakers would be:
* A stage based on Tokyo Mirage Sessions that is literally the musical stage from the battles in that game, with Mirages appearing as hazards.
* A Xenoblade X stage based on New Los Angeles with Skells flying around in the background.
* Ice Climber's stage from Brawl returns.