THE GUEST FIGHTERS
Just lumping them into their own category since the inclusion of (at least) one more company to negotiate with means they are more complicated to properly guess. Some highly beloved characters could be cut while some less desirable ones could return. It all depends on who Nintendo wants to play ball with and which companies accept the invite to play ball.
With that said, Sonic being the only guest to appear in more than just one Smash game (discounting Ultimate) makes him likely to return while the fact that Square Enix almost single-handedly stopped "Everyone is Here!" by making it hard to negotiate Cloud means any of
their characters have shaky chances in spite of their popularity.
They're basically wild cards.
THE UNTOUCHABLE TEN
These are the characters that feel too big when it comes to Nintendo's own IPs, carrying a lot of long-lasting legacy and even being at the very least planned in Smash 64 so Sakurai has had his sights on them since Day 1.
As a result, seeing Smash without any of these ten characters would be ridiculous imo.
Not
all of Smash 64's roster is there but you could say that's a result of times changing. Some franchises aren't what they were like in '99.
SMASH LEGACY PICKS
Characters who are absolutely more famous for being in Smash than they are for their source material. That fame can help with their likeliness and if any of them get cut, they remain a good contender for DLC due to that very fame. Some of them such as Fox or Ness could also fit more categories but I think this would be the reason why any of them would return.
With that said, if Dr. Mario is one of those cuts, I doubt he'll be sold as his own standalone DLC.
THE (FIRST-PARTY) SMASH BALLOT PICKS
First-party characters explicitly known to have made it in thanks to the Smash Ballot, meaning their position as characters with high fan demand might warrant them being brought back. The two Echo Fighters are in a more questionable position since we don't know if the next game will bring back Echo Fighters but if it does, they're going to be pretty high on the returning Echo Fighters list.
Realistically, K. Rool and Ridley could also both be in the "Legendary All-Stars" category and Chrom could be either grouped with the Echo Fighters or be considered unique enough of a clone to stand among the "Rising All-Stars" but I felt the Ballot had a lot of weight in their inclusion and should remain acknowledged.
Speaking of Echo Fighters...
THE ECHO FIGHTERS
Highly dependent on if they bring the concept back. And while I don't want to guess the priority of each, I would say Lucina is likely one of the highest ones due to how the lack of tipper hitboxes make her feel completely distinct despite being a full-on clone. So distinct that I think she and Chrom can still return even if the concept of Echoes doesn't; they feel THAT unique.
NINTENDO'S LEGENDARY ALL-STARS
Characters who are popular even outside of Smash and have had a long-lasting legacy that predates the Wii (which I'm using as the cutoff point for legacy here since GameCube games are getting more remakes lately).
These characters are likely to be very high on the priority list but not 100% guaranteed. If they make a lot of cuts for the next game, I could some of them removed but if it reaches beyond half of them, I expect the number of returning veterans to be outright nuked. However, if any of them are gone, it will absolutely be time constraints.
NINTENDO'S RISING ALL-STARS
This is mostly odern all-stars from after the GameCube era, though long-standing characters who saw a rising
(pun intended) prominence after that era after getting a new game during or after the Wii era are also present here (this is mostly for Kid Icarus but Punch-Out is there too).
They very much have similar likeliness as the category about but I wanted to give them their own category because they generally don't carry as much legacy and instead have the opportunity to build even more legacy as the years go by.
Like the above category, time constraints is more likely to do them in than low priority.
THE OTHERS
These are the ones I have no idea where to put for various reasons.
Plant was very much Sakurai being silly and we don't know if the joke will stick again. Young Link could fit as a legacy pick but also three Links is a bit much and Toon is more visually distinct. Pichu was always a joke addition. And the latter two are basically Echoes in all but official acknowledgement.
Finally, Lucas has the "honor" of being the only character whose sole game appearance outside of Smash is from a Japan-exclusive game but "Release Mother 3" is such a meme that I can't just say he's exclusively Smash legacy... yet this Japan-exclusivity is also why I doubt he'll return.