I'll give a go at seven newcomers...
1. King K. Rool. Based on what little we know of the ballot, it's really likely he did well. He's been acknowledged with a trophy every Smash game so far, so it's not like he's impossibly obscure, and the Mii costume suspiciously came out of nowhere - we wouldn't have gotten that unless Nintendo saw how well he was doing on the ballot.
2. Decidueye still seems like the most likely Gen 7 mon to me. Mimikyu'd make an excellent Pokéball summon, and Lycanroc's concept art leaves a lot to be desired. Incineroar could be competition, but I think Sakurai'd like that Deci's got more unique potential than a mon who could easily be considered 'Captain Falcon clone'
3. Chorus Kids, or at least something from Rhythm Heaven. Gematsu's basically confirmed as an early roster draft, so I can see them revisiting this. I'd prefer the Wrestler if given a choice since that game is kinda fun/ny.
4. Geno's been big. He's got a huge fanbase and Sakurai likes him. See K.Rool with the Mii outfit.
5. Elma's another name that's being thrown about a lot based on Xeno. If Nintendo is trying to promote that series, they'd probably want it to be one of the few non-original 12 series that has multiple reps.
6. Ashley, since she's got massive support in Japan and has pretty well become the face of WarioWare by now.
7. Rayman is at the top of my 'most likely third parties' list based on his inexplicable trophy in Sm4sh and the hype surrounding the Artsy Omni thing. I bet he did unexpectedly well because of that. Simon is on everyone's lips now due to Vergeben, and BK had a lot of support after Phil Spencer tweeted his support ("No, I don't want a bunch of free advertising for my IP on a competitor's system..." of course he wants it!), but I don't know. I'd love to be able to put Dovahkiin in this slot with the hope that Todd Howard requested it when porting Skyrim to the Switch, but that's just fantasy. Point is, I'm not expecting more than two vanilla 3rd parties, incuding Geno, based on who's already in.
I could also see Isabelle as a Villager echo and Dixie as an echo or semi-clone of Diddy. Impa (Shiek) and Chrom (Ike) are also echoes that they could do.
For DLC, I'd guess Rex/Pyra (if Elma's not in) and Spring Man are already being considered. I wonder if there'll be another ballot specifically for DLC...the last one said it was for DLC, but the only character we got in from the ballot was made ground-up way too quickly to be believably from the ballot alone.