Going off of leaks and speculation/popularity I think these will probably be the new comers in Ultimate.
1) Bandana Waddle Dee - Thanks to Memoryman3 hacking the 3DS version of Yoshi’s Wooly World, we know there are two Amiibos coming soon, one being another Daisy, while the other is another Waddle Dee. While Memoryman3 is a huge Daisy fanboy, that doesn’t make him uncredible, he actually did call Daisy being in Smash through this leak, and the fact that he also mentioned Waddle Dee makes me think Bandana Dee may be a lock for Smash. Other things going for Bandana Dee include him being the only Kirby character left out of the Return to Dreamland 4 and what looks to be Sakurai filling gaps in the roster, ie Ridley and K Rool joining as Metroid and DK were missing their main villains, and Chrom completing the Awakening trio. It also helps that Bandana Waddle Dee was a popular ballot pick, ranking 4th in Japan behind K Rool and the two returning Brawl veterans, Snake and Wolf.
2) Skull Kid - Sakurai set his room up with pillows that look like Skull Kid’s shirt, and two chairs, one purple and one yellow, which could represent the fairies. If you didn’t know, the E3 direct had Sakurai in a room with barrels and a more wooden atheistic, which probably hinted at K Rool. Other evidence is the lack of his assist trophy, which is seemingly being replaced by the moon, a mini costume, and the idea of the villains theme, which means Skull Kid would be the villain to Young Link. Skull Kid also has a massive following in the Zelda community and Majora’s Mask is iconic and very recognizable as a component of Zelda. It also helps that people want a new Zelda character that isn’t Link or Zelda.
3) Steve - Yes, the avatar from Minecraft will most likely make it. Verge apparently has heard something about Minecraft content in Smash. While that doesn’t necessarily mean a character, my thinking is, why would you include content from the biggest game of the decade, but not a character. Steve would make complete sense, MC has become a gaming icon worthy of standing next to Mario. Minecraft represents a lot, with one being Indies which people voted for on the Ballot, but unlike Shovel Knight, Minecraft has become more of a house-hold name to deserve the indie spot. Steve would also be extremely unique, representing world building games, a genre yet to be seen in Smash, and he has basically become the face of Microsoft. Finally with stuff like cross-play, the 3DS title and exclusive 3DS, Nintendo is shown to be pushing the series hard probably because Steve will make a big debut in Smash.
4) Gardevior - The Richter leak called it, but it makes sense. People probably want every generation represented, so why take the risk and develop a new Pokémon that people may not like, when you can instead go back to the past and complete the generations. The Gothitelle echo may not be the best, but at least the generations can be complete for once.
5) Issac - He is popular and has been requested for years. His franchise is also at a loss for any representation other than his assist in Brawl and some music, so I think with popularity in mind, he has a pretty high chance.
6) Isabelle (Echo) - Verge called it, that is all. JK, no but in all seriousness, Isabelle would be an easy echo for Villager, and sense she’s become an icon for the franchise, and with Animal Crossing being huge after her game New Leaf, then I think it’s safe to say we’ll be seeing Isabelle.
7) Shadow (Echo) - They gave Castlevania two characters, and we all know Sonic is way more popular, so Sonic has to get a second rep. Since they most likely don’t want to make a fully new character, than Shadow is the best bet as he’s popular and was a clone of Sonic anyway. Plus with Chrom confirming that echoes can be original, maybe he can have Chaos powers.
8) Dixie Kong (Echo) - Remember when I said that Sakurai seems to be filling gaps, well, this would apply to Dixie. Being the last main Kong, I can’t see them including K Rool and excluding her. These 4 made the original DKC titles, and with Tropical Freeze seemingly being aware of Dixie’s popularity in its marketing, why would you leave out this popular third wheel. It also helps that once again, Echoes can be original, so she could have her hair move, and she was planned for Brawl, so the idea is there.
9) Octolings - They are the easiest set of Echoes you could make, as they are just Inklings. They would also be a modern villain to add to the villains trend. Add on that they are popular in the Splatoon community, and they would make for a great echo.