If Nintendo is picking the last 6, I'd imagine they're going to be all in-house, promotional characters. They have no reason to pay yet another 3rd party developer for the rights to use a character. In they're eyes, they've probably overloaded it enough with third party. I could see maybe another Sega character at most since Nintendo is pretty close with them and may not need to bargain much for another Sonic character or something.
As for the others, likely all Promotional characters depending on the release slate in the comings months (year):
A Pokemon character to help drum up more attention for the Crown & Tundra expansion to be added to Gen 8. In terms of likeliness for a Gen 8 pokemon (if a pokemon is indeed added), this would be most likely to least I think (least as in, at least has a chance): Rillaboom, Intelleon, Cinderace, Grimmsnarl, Urishifu,Toxtricity, Duralduron, new Legendary pokemon whose Silhouette was recently shown off. I'm 100% that if we get a new Pokemon rep, then it will be a Gen 8 pokemon.
A Zelda character; likely Zelda herself; from BotW 2 after the game gets more detail released during the Summer months when Nintendo does a huge Direct info dump. Impa or a new Ganon are also possible; I think if we get another Zelda rep, its going to be Either Zelda, Ganon, or Impa (three highly recurring characters) as opposed to someone like Midna in her real form or Majora (as much as I'd like for Majora to be added; unless the new game has Majora come back, I don't see them adding the character in now of all times).
Fiora. We've already got Shulk, people have been clamoring for Fiora, and we've got the Definitive Edition coming soon; I wouldn't put it past them to add her to help promote the definitive edition coming.
Another Metroid character. This could possibly be the greatest iteration for Smash brothers with a 3rd Metroid character added since they've got Metroid Prime 4 coming and may look to add someone from the game to hype it up a bit.
Another Mario franchise character. Mario is their flagship series and is the face of Nintendo (even though pokemon and Zelda are both enormous faces of Nintendo as well). We'll undoubtedly learn of a new Mario game in the coming months (since Odyssey has been out for awhile now) and I'd say its a good chance we get someone who somehow hasn't gotten representation yet (like King Boo or Kamek or something) or possibly even a new villain. Mario Odyssey was announced with, what, a 3 month wait time until the game actually released? They kept that pretty well under wraps. Come this summer, I think we're going to get a whole slew of announcements for core Nintendo titles where they will have plenty of potential for Smash characters to help hype up the release.
Final possibility would be an 'old school' character like Ryu or one of their very old, nearly forgotten properties.
This is just my guess/take on it since we learned Nintendo fully picked the characters (with Sakurai's only input likely being whether the character would be do-able or not). I sincerely doubt I'm 100% correct on my guess here, but I definitely expect to see at least one character from the possible franchises I named to make it in as one of the last 6 DLC additions.